Semiconductor device and method of manufacturing the same

ABSTRACT

A semiconductor device with high reliability and operation performance is manufactured without increasing the number of manufacture steps. A gate electrode has a laminate structure. A TFT having a low concentration impurity region that overlaps the gate electrode or a TFT having a low concentration impurity region that does not overlap the gate electrode is chosen for a circuit in accordance with the function of the circuit.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of U.S. application Ser.No. 10/793,031, filed Mar. 5, 2004, now U.S. Pat. No. 6,828,586, whichis a divisional of U.S. application Ser. No. 10/456,608, filed Jun. 9,2003, now U.S. Pat. No. 6,707,068, which is a divisional of U.S.application Ser. No. 09/916,329, filed Jul. 30, 2001, now U.S. Pat. No.6,613,620, which claims the benefit of foreign priority applicationsfiled in Japan as Serial No. 2000-230401 on Jul. 31, 2000, as Serial No.2000-301389 on Sep. 29, 2000, and as Serial No. 2000-301390 on Sep. 29,2000. This application claims priority to each of these priorapplications, and the disclosures of the prior applications areconsidered part of (and are incorporated by reference in) the disclosureof this application.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor device having a circuitcomprising a thin film transistor TFT that uses a crystallinesemiconductor film formed on a substrate (a liquid crystal displaydevice, in particular), and to a method of manufacturing thesemiconductor device. The semiconductor device manufactured inaccordance with the present invention is specifically a liquid crystaldisplay device represented by an active matrix liquid crystal displaydevice in which a pixel portion and a driver circuit to be placed in theperiphery of the pixel portion are formed on the same substrate. Theinvention also relates to electronic appliances that employ the displaydevice as a display unit.

2. Description of the Related Art

TFTs with a crystalline semiconductor film (typically, a polysiliconfilm) on an insulating surface as a semiconductor element are used forvarious integrated circuits at present. The TFTs are used most often asswitching elements of a display device. The TFTs having, as an activelayer (a semiconductor layer including a channel formation region, asource region, and a drain region), a crystalline semiconductor film,which provides higher mobility than an amorphous semiconductor film arehigh in driving performance, and hence used also as elements of a drivercircuit. Accordingly, in an active matrix liquid crystal display device,for example, an image circuit for displaying an image and a drivercircuit for controlling the image circuit are formed on a singlesubstrate.

In an active matrix liquid crystal display device, integrated circuitssuch as a pixel circuit for displaying an image, a shift registercircuit based on a CMOS circuit, a level shifter circuit, a buffercircuit, and a sampling circuit are all arranged on a single substratewhile forming different functional blocks. A liquid crystal displaydevice as above has excellent features including being thin,small-sized, lightweight, and low in power consumption. For that reason,the liquid crystal display device is now used in various scenes; to namea few, as a display unit of a personal computer for space saving and asa display unit of a portable information equipment for obtaining thelatest information anytime, any place.

A pixel portion of the liquid crystal display device has a TFTfunctioning as a switching element (also called a pixel TFT) and astorage capacitor, and is driven by applying a voltage to a liquidcrystal. The liquid crystal has to be driven with an alternate current,and a method called frame inversion driving is often employed. The TFTis required to have a characteristic of sufficiently low OFF current(Ioff: the value of drain current flowing when the TFT is inoff-operation). However. OFF current is often high when the TFT isformed of a polysilicon film. A known solution for this problem is theLDD structure with a low concentration impurity region (lightly dopeddrain) (a structure in which an impurity region having a lowconcentration is provided between a channel formation region and asource region or a drain region doped with a high concentration ofimpurity element).

On the other hand, high driving voltage is applied to a buffer circuitand the circuit needs to have a withstand voltage high enough to preventdamage against high voltage. In order to enhance the current drivingability, the ON current value has to be sufficiently high (Ion: thevalue of drain current flowing when the TFT is in on-operation).Degradation of the ON current value due to hot carriers is effectivelyprevented by a known structure called the GOLD (gate-drain overlappedLDD) structure in which a gate electrode partially overlaps an LDDregion (with a gate insulating film interposed therebetween).

In order to obtain a semiconductor device that meets the requiredperformance, it is necessary to fabricate different TFTs for differentcircuits. However, increased number of masks are needed to form an LDDstructure TFT and a GOLD structure TFT. An increase in number of masksused leads to more manufacture steps, complication of the manufactureprocess, and reduction in yield.

SUMMARY OF THE INVENTION

The present invention has been made in view of the above, and an objectof the present invention is therefore to provide a semiconductor device,typically, an active matrix liquid crystal display device, in which OFFcurrent of a TFT in a pixel portion is reduced and the reliability of aTFT in a driver circuit is improved (i.e., degradation due to hotcarriers is reduced) without increasing the number of masks.

A liquid crystal display device is low in light utilization efficiencyand front light or back light is often used during display in order toimprove visibility. The use of front light or back light raises powerconsumption of its display portion, canceling low power consumption ofthe liquid crystal display device itself. Accordingly, another object ofthe present invention is to provide a display device of excellentvisibility without increasing the number of manufacture steps.

According to the present invention, there is provided a semiconductordevice having a TFT that is formed in a pixel portion and an n-channelTFT and a p-channel TFT that constitute a driver circuit provided in theperiphery of the pixel portion, all of the TFTs being formed on the samesubstrate, characterized in that the n-channel TFT has a secondconcentration impurity region that partially overlaps a gate electrode,and that the p-channel TFT and the TFT formed in the pixel portionrespectively have second concentration impurity regions that do notoverlap grate electrodes.

According to the present invention, there is provided a semiconductordevice having a TFT that is formed in a pixel portion and an n-channelTFT and a p-channel TFT that constitute a driver circuit provided in theperiphery of the pixel portion, all of the TFTs being formed on the samesubstrate, characterized in that the n-channel TFT has a gate electrodecomposed of a first conductive film and a second conductive film, thefirst conductive film contacting the top face of a gate insulating film,the second conductive film contacting the top face of the firstconductive film, the first conductive film being longer than the secondconductive film in the channel length direction, the first conductivefilm partially overlapping a second concentration impurity region, andcharacterized in that the p-channel TFT and the TFT formed in the pixelportion respectively have gate electrodes that do not overlap secondconcentration impurity regions, the gate electrodes being composed ofthe first conductive film that contacts the top face of the gateinsulating film and the second conductive film that contacts the topface of the first conductive film, the first conductive film and thesecond conductive film having the same length in the channel lengthdirection.

According to the present invention, there is provided a semiconductordevice having a driver circuit that is composed of an n-channel TFT, afirst p-channel TFT, and a second p-channel TFT, characterized in that:the n-channel TFT has a semiconductor layer, a gate insulating filmformed on the semiconductor layer, and a gate electrode formed on thegate insulating film, the semiconductor layer including a channelformation region, a source region, a drain region, and a secondconcentration impurity region: the gate electrode is composed of a firstconductive film and a second conductive film, the first conductive filmcontacting the top face of the gate insulating film, the secondconductive film contacting the top face of the first conductive film:the second concentration impurity region overlaps the first conductivefilm with the gate insulating film interposed therebetween; the firstp-channel TFT has a semiconductor layer, a gate insulating film formedon the semiconductor layers and a gate electrode formed on the gateinsulating film, the semiconductor layer including a channel formationregion, a source region, a drain region, and a fifth concentrationimpurity region the channel formation region and the gate electrode ofthe first p-channel TFT have substantially the same length in thechannel length direction: the second p-channel TFT has a semiconductorlayer, a gate insulating film formed on the semiconductor layer, and agate electrode formed on the gate insulating film, the semiconductorlayer including a channel formation region, a source region, a drainregion, and a fifth concentration impurity region; the gate electrode ofthe second p-channel TFT is composed of a first conductive film and asecond conductive film, the first conductive film contacting the topface of the gate insulating film, the second conductive film contactingthe top face of the first conductive film: and the fifth concentrationimpurity region of the second p-channel TFT overlaps the firstconductive film with the gate insulating film interposed therebetween.

According to the present invention, there is provided a semiconductordevice having a driver circuit that is composed of an n-channel TFT, afirst p-channel TFT, and a second p-channel TFT, characterized in that:the n-channel TFT has a semiconductor layer, a gate insulating filmformed on the semiconductor layer, and a gate electrode formed on thegate insulating film, the semiconductor layer including a channelformation region, a source region, a drain region, and a secondconcentration impurity region; the gate electrode is composed of a firstconductive film and a second conductive film, the first conductive filmcontacting the top face of the gate insulating film, the secondconductive film contacting the top face of the first conductive film:the second concentration impurity region overlaps the first conductivefilm with the gate insulating film interposed therebetween; the firstp-channel TFT has a semiconductor layer, a gate insulating film formedon the semiconductor layer, and a gate electrode formed on the gateinsulating film, the semiconductor layer including a channel formationregion, a source region, a drain region, a fifth concentration impurityregion and an offset region; the second p-channel TFT has asemiconductor layer, a gate insulating film formed on the semiconductorlayer, and a gate electrode formed on the gate insulating film, thesemiconductor layer including a channel formation region, a sourceregion, a drain region, and a fifth concentration impurity region; thegate electrode of the second p-channel TFT is composed of a firstconductive film and a second conductive film, the first conductive filmcontacting the top face of the gate insulating film, the secondconductive film contacting the top face of the first conductive film;and the fifth concentration impurity region of the second p-channel TFToverlaps the first conductive film with the gate insulating filminterposed therebetween.

According to the present invention, there is provided a semiconductordevice having a driver circuit and a pixel portion, the driver circuitbeing composed of an n-channel TFT, a first p-channel TFT, and a secondp-channel TFT, the pixel portion including a TFT and a storagecapacitor, characterized in that: the n-channel TFT has a semiconductorlayer, a gate insulating film formed on the semiconductor layer, and agate electrode formed on the gate insulating film, the semiconductorlayer including a channel formation region, a source region, a drainregion, and a second concentration impurity region; the gate electrodeis composed of a first conductive film and a second conductive film, thefirst conductive film contacting the top face of the gate insulatingfilm, the second conductive film contacting the top face of the firstconductive film; the second concentration impurity region overlaps thefirst conductive film with the gate insulating film interposedtherebetween: the first p-channel TFT has a semiconductor layer, a gateinsulating film formed on the semiconductor layer, and a gate electrodeformed on the gate insulating film, the semiconductor layer including achannel formation region, a source region, a drain region, a fifthconcentration impurity region, and an offset region: the secondp-channel TFT has a semiconductor layer, a gate insulating film formedon the semiconductor layer, and a gate electrode formed on the gateinsulating film, the semiconductor layer including a channel formationregion, a source region, a drain region, and a fifth concentrationimpurity region: the gate electrode of the second p-channel TFT iscomposed of a first conductive film and a second conductive film, thefirst conductive film contacting the top face of the gate insulatingfilm, the second conductive film contacting the top face of the firstconductive film; the fifth concentration impurity region of the secondp-channel TFT overlaps the first conductive film with the gateinsulating film interposed therebetween: and the TFT formed in the pixelportion has a semiconductor layer that includes a channel formationregion, a source region, a drain region, a second impurity region, andan offset region.

Further, according to the present invention, there is provided asemiconductor device having a driver circuit that is composed of ann-channel TFT, a first p-channel TFT, and a second p-channel TFT,characterized in that: the n-channel TFT has a semiconductor layer, agate insulating film formed on the semiconductor layer, and a gateelectrode formed on the gate insulating film, the semiconductor layerincluding a channel formation region, a source region, a drain region,and a second concentration impurity region: the gate electrode iscomposed of a first conductive film and a second conductive film, thefirst conductive film contacting the top face of the gate insulatingfilm, the second conductive film contacting the top face of the firstconductive film: the second concentration impurity region has an L_(ov)region and an L_(off) region, and the L_(ov) region overlaps the firstconductive film with the gate insulating film interposed therebetweenwhereas the L_(off) region does not overlap the first conductive film;and the first p-channel TFT and the second p-channel TFT respectivelyhave semiconductor layers, each of the semiconductor layers including achannel formation region, a source region, a drain region, and a fifthconcentration impurity region.

In the above present invention, the semiconductor device ischaracterized in that the gate electrodes of the n-channel TFT, thep-channel TFTs, and the TFT formed in the pixel portion are formed of anelement selected from the group consisting of Ta, W, Ti, Mo, Al, and Cu,or formed of an alloy material or a compound material containing anyelement in the group above as its main ingredient.

In the above present invention, the semiconductor device ischaracterized in that a plurality of protrusions are formed in the pixelportion: the TFT formed in the pixel portion is electrically connectedto a pixel electrode that is uneven: and the uneven portion of the pixelelectrode has a radius of curvature of 0.1 to 0.4 μm, and the unevenportion of the pixel electrode is 0.3 to 3 μm tall.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIGS. 1A to 1C are diagrams showing an embodiment mode of the presentinvention;

FIGS. 2A to 2C are diagrams showing the embodiment mode of the presentinvention;

FIGS. 3A to 3C are diagrams showing a process of manufacturing asemiconductor device according to the present invention;

FIGS. 4A to 4C are diagrams showing a process of manufacturing asemiconductor device according to the present invention;

FIGS. 5A and 5B are diagrams showing a process of manufacturing asemiconductor device according to the present invention;

FIG. 6 is a diagram showing the structure of a semiconductor deviceaccording to the present invention;

FIGS. 7A to 7C are diagrams showing a process of manufacturing asemiconductor device according to the present invention;

FIGS. 8A to 8C are diagrams showing a process of manufacturing asemiconductor device according to the present invention;

FIGS. 9A to 9C are diagrams showing a process of manufacturing asemiconductor device according to the present invention;

FIG. 10 is a diagram showing the top view of a semiconductor deviceaccording to the present invention;

FIG. 11 is a diagram showing a sectional view of a semiconductor deviceaccording to the present invention;

FIGS. 12A and 12B are diagrams showing a process of manufacturing asemiconductor device according to the present invention;

FIGS. 13A and 13B are diagrams showing a process of manufacturing asemiconductor device according to the present invention;

FIGS. 14A to 14C are diagrams showing a process of manufacturing asemiconductor device according to the present invention;

FIGS. 15A to 15C are diagrams showing a process of manufacturing asemiconductor device according to the present invention;

FIGS. 16A to 16C are diagrams showing a process of manufacturing asemiconductor device according to the present invention;

FIG. 17 is a diagram showing a process of manufacturing a semiconductordevice according to the present invention;

FIGS. 18A to 18C are diagrams showing a process of manufacturing asemiconductor device according to the present invention;

FIGS. 19A to 19C are diagrams showing a process of manufacturing asemiconductor device according to the present invention;

FIGS. 20A to 20C are diagrams showing a process of manufacturing asemiconductor device according to the present invention;

FIGS. 21A to 21C are diagrams showing a process of manufacturingsemiconductor device according to the present invention;

FIG. 22 is a diagram showing a sectional view of a semiconductor deviceaccording to the present invention;

FIG. 23 is a circuit block diagram of an active matrix liquid crystaldisplay device;

FIG. 24 is a circuit block diagram of an active matrix liquid crystaldisplay device;

FIGS. 25A to 25D are diagrams showing an exemplary method ofcrystallizing a semiconductor film;

FIGS. 26A to 26D are diagrams showing an exemplary method ofcrystallizing a semiconductor film;

FIGS. 27A and 27B are diagrams showing a process of manufacturing asemiconductor device according to the present invention;

FIGS. 2SA to 28C are diagrams showing a process of manufacturing asemiconductor device according to the present invention;

FIGS. 29A and 29B are diagrams showing a process of manufacturing asemiconductor device according to the present invention;

FIG. 30 is a top view of a semiconductor device according to the presentinvention;

FIGS. 31A and 31B are diagrams showing the circuit structure of an EEMOScircuit and an EDMOS circuit, respectively;

FIG. 32 is a graph showing results of measuring the reliability of a TFTmanufactured in accordance with the present invention;

FIG. 33 is a graph showing results of measuring the reliability of a TFTmanufactured in accordance with the present invention;

FIGS. 34A to 34F are diagrams showing examples of an electronicappliance;

FIGS. 35A to 35D are diagrams showing examples of an electronicappliance;

FIGS. 36A to 36C are diagrams showing examples of an electronicappliance;

FIG. 37 is a graph showing the Id-Vg curve of a TFT manufactured inaccordance with the present invention;

FIG. 38 is a graph showing the Id-Vg curve of a TFT manufactured inaccordance with the present invention;

FIG. 39 is a diagram showing a sectional view of an inverter circuit;

FIG. 40 is a graph showing the Id-Vg curve of a STe manufactured inaccordance with the present invention;

FIGS. 41A and 41B are graphs showing the Id-Vg curve of TFTsmanufactured in accordance with the present invention;

FIG. 42 is a graph showing results of measuring the reliability of a TFTmanufactured in accordance with the present invention;

FIG. 43 is a graph showing results of measuring the reliability of a TFTmanufactured in accordance with the present invention;

FIG. 44 is a graph showing results of measuring the reliability of a TFTmanufactured in accordance with the present invention;

FIGS. 45A and 45B are graphs showing results of measuring thereliability of TFTs manufactured in accordance with the presentinvention;

FIG. 46 is a diagram showing an embodiment of the present invention; and

FIGS. 47A and 47B are respectively a top view and a sectional view of anembodiment of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS Embodiment Mode 1

An embodiment mode of the present invention will be described withreference to FIGS. 1A to 2C.

On a substrate 10, a base insulating film 11 is formed from aninsulating film such as a silicon oxide film, a silicon nitride film,and a silicon oxynitride film. The base insulating film 11 in thisembodiment mode has a two-layer structure 11 a and 11 b. However, thebase insulating film may have a single layer or three or more layers ofthe insulating films given in the above.

Next, an amorphous semiconductor film is formed on the base insulatingfilm 11 to a thickness of 30 to 60 nm. No limitation is put on thematerial of the amorphous semiconductor film, but the film is preferablyformed of silicon or a silicon germanium (Si_(x)Ge_(1−x); 0<x<1,typically x=0.001 to 0.05) alloy. The amorphous semiconductor film isthen subjected to a known crystallization treatment (such as lasercrystallization thermal crystallization, or thermal crystallizationusing nickel or other catalysts) to form a crystalline semiconductorfilm. The obtained crystalline semiconductor film is patterned into adesired shape to form semiconductor layers 12 to 14.

After forming the semiconductor layers 12 to 14, the layers may be dopedwith an impurity element imparting p-type conductivity in order tocontrol the threshold of an n-channel TFT. Known impurity elements thatcan give a semiconductor the p type conductivity are elements belongingto Group 13 in the periodic table, such as boron (B), aluminum (Al), andgallium (Ga).

A gate insulating film 15 is formed next to cover the island-likesemiconductor layers 12 to 14. The gate insulating film 15 is formed byplasma CVD or sputtering from an insulating film containing silicon tohave a thickness of 40 to 150 nm. Of course, the gate insulating filmmay be a single layer or a laminate of an insulating film containingsilicon.

Subsequently formed on the gate insulating film 15 as a laminate are afirst conductive film (TaN film) 16 a with a thickness of 20 to 100 nmand a second conductive film (W film) 16 b with a thickness of 100 to400 nm. The conductive films may be formed of an element selected fromthe group consisting of Ta, W, Ti, Mo, Al, and Cu, or formed of an alloymaterial or a compound material containing any element in the groupabove as its main ingredient. Alternatively, a semiconductor film,typically a polycrystalline silicon film, doped with an impurity elementsuch as phosphorus may be used.

Next, resist masks 17 to 19 are formed by photolithography and a firstetching treatment is conducted by ICP (inductively coupled plasma)etching or other etching methods to form an electrode and a wiring line.The W films 20 b to 22 b are first etched under first etching conditionsto taper the first conductive film around the edge, and then the W films20 b to 22 b and the TaN films 20 a to 22 a are simultaneously etchedunder second etching conditions to form first shape conductive layers 20to 22. Denoted by 26 is a part of the gate insulating film, and regionsthereof that are not covered with the first shape conductive layers 20to 22 are also etched and thinned.

Then a first doping treatment is conducted, without removing the resistmasks to dope the semiconductor layers with an impurity elementimparting n-type conductivity. Ion doping or ion implantation isemployed for the doping treatment. In the first doping treatment, thefirst shape conductive layers 20 to 22 serve as masks against theimpurity element imparting n-type conductivity to form firstconcentration impurity regions 23 to 25 in a self-aligning manner.

Still keeping the resist masks in place, a second etching treatment isconducted as shown in FIG. 1C. Second shape second conductive films 2 bto 29 b are formed by anisotropic etching. At this point, the firstconductive layers and the gate insulating film are also etched slightlyto form second shape first conductive films 27 a to 29 a. As a result,second shape conductive layers 27 to 29 (the first conductive films 27 ato 29 a and the second conductive films 27 b to 29 b) and a gateinsulating film 39 are formed.

A second doping treatment is next conducted without removing the resistmasks. In the second doping treatment, the layers are doped with animpurity element imparting n-type conductivity in a dose smaller than inthe first doping treatment and at a high acceleration voltage. Thusformed are second concentration impurity regions 33 to 35 and 36 to 38that are newly formed in the semiconductor layers inside the firstconcentration impurity regions formed in FIG. 1B. In the doping, thesemiconductor layers under the second shape first conductive films 27 ato 29 a are also doped with the impurity element while using the secondshape conductive layers 27 to 29 as masks.

Thus formed are third concentration impurity regions 36 to 38 and secondconcentration impurity regions 33 to 35. The third concentrationimpurity regions 36 to 38 overlap the second shape first conductivefilms 27 a to 29 a respectively. The second concentration impurityregions are placed between the first concentration impurity regions andthe third concentration impurity regions (33 is between 30 and 36, 34 isbetween 31 and 37, and 35 is between 32 and 38).

Then the resist masks are removed. Thereafter, a mask 40 is newly formedfrom a resist so as to cover the n-channel TFT of a driver circuitportion. A third etching treatment is conducted as shown in FIG. 2A. Thefirst conductive layers of a p-channel TFT and of the TFT in the pixelportion are etched to form third shape conductive layers 41 and 42. Atthis point, a gate insulating film 43 that is not covered with the mask40 is slightly etched and thinned.

In order to avoid fluctuation caused by the uneven gate insulating film,the gate insulating film is etched after the resist mask is removed asshown in FIG. 2B. The conductive layers serve as masks to leave portionsof the gate insulating film unetched, thereby forming gate insulatinglayers 44 to 46.

Next, resist masks 47 and 48 are newly formed to conduct a third dopingtreatment as shown in FIG. 2B. In the third doping treatment, thesemiconductor layer to be an active layer of the p-channel TFT is dopedwith an impurity element imparting p-type conductivity while using thethird shape conductive layer 41 (41 a and 41 b) as a mask against theimpurity element. As a result, fourth concentration impurity regions 49to 51 are formed in a self-aligning manner.

In this way, TFTs shown in FIG. 2C are manufactured. An n-channel TFT 71of a driver circuit 73 includes: the third concentration impurity region36 overlapping with the second shape conductive layer 27 for forming agate electrode (the region 36 is called a GOLD region in thisspecification); the second concentration impurity region 33 formedoutside the gate electrode (the region 33 is called an LDD region inthis specification); and the first concentration impurity regionfunctioning as a source region or a drain region. A reference symbol 72denotes a p-channel TFT of the driver circuit 73. A pixel TFT 74 in thepixel portion has the third concentration impurity region 38 and thesecond concentration impurity region 35 formed outside the gateelectrode (the regions 38 and 35 are both called LDD regions in thisspecification), and has the first concentration impurity region 32functioning as a source region or a drain region.

Embodiment Mode 2

This embodiment mode describes a method of forming an uneven electrodewith projections formed by the same process that is used to form a TFTin a pixel portion.

A substrate is prepared by forming an insulating film on a surface of aglass substrate, a quartz substrate, a silicon substrate, a metalsubstrate, or a stainless steel substrate. A plastic substrate may alsobe used as long as it has a heat resistance against the processtemperature of embodiments. A base insulating film is formed on thesubstrate and a semiconductor layer is formed on the base insulatingfilm.

The projections can have high reproducibility when formed using a photomask. Therefore it is appropriate to form the projections in accordancewith the process of manufacturing a pixel TFT 103. An example of formingthe projections by layering the semiconductor layer, a gate insulatingfilm, and a conductive film similar to the manufacture of the pixel TFT1203 is illustrated in FIGS. 5A to 5B.

The method of forming the projections is not particularly limited and asingle layer of one of the above films, or a laminate combining theabove films may be used. For example, the projections may be a laminateof the semiconductor layer and the insulating film, or a single layer ofthe conductive film. In other words, a plurality of projections can beformed without increasing the number of steps for manufacturing asemiconductor device.

The thus formed projections, as well as the pixel TFT formed by the sameprocess and a TFT in a driver circuit, are covered with an interlayerinsulating film. The curvature of the uneven portion of the pixelelectrode can be adjusted by selecting the material of the insulatingfilm. The radius of curvature of the uneven portion of the pixelelectrode is set to 0.1 to 0.4 μm (preferable 0.2 to 2 μm). When theinsulating film is an organic resin film, an appropriate organic resinfilm has a viscosity of 10 to 1000 cp (preferably 40 to 200 cp) (forinstance, a polyimide film or an acrylic resin film), so that thesurface of the film shows enough irregularities in accordance with theunderlying uneven region.

After forming the uneven interlayer insulating film, the pixel electrodeis formed thereon. The surface of the pixel electrode is also irregulardue to the uneven insulating film. The uneven portion is 0.3 to 3 μmtall. With the uneven portion formed on the surface of the pixelelectrode, light can effectively be scattered when incident light isreflected as shown in FIG. 6.

The projections shown in this embodiment mode are a laminate of thesemiconductor layer, the gate insulating film, the first conductivefilm, and the second conductive film layered in accordance with theprocess of manufacturing the pixel TFT. However, the projections are notparticularly limited and any layer or film given in the above can form asingle layer or a laminate to serve as the projections. Thus theprojections having a necessary height can be formed without increasingthe number of manufacture steps. One projection is spaced apart from anadjacent projection by 0.1 μm or more, preferably 1 μm.

The projections desirably vary in size in order to scatter the reflectedlight better, though no particular limitation is set. The shape andarrangement of the projections may irregular or regular. Furthermore,the projections do not need to be in any particular place as long asthey are in a region below the pixel electrode which corresponds to thedisplay region of the pixel portion.

An appropriate size of the protrusion when viewed from the above is 100to 400 m², preferably 25 to 100 μm².

In this way, the uneven pixel electrode can be formed without increasingthe number of manufacture steps.

Embodiment 1

Embodiments of the present invention will be described with reference toFIGS. 7A to 11. Here, a detailed description will be given on a methodof forming, simultaneously, on the same substrate, a TFT for a pixelportion and TFTs (an n-channel TFT and a p-channel TFT) for a drivercircuit that is provided in the periphery of the pixel portion.

A substrate 100 may be a glass substrate, a quartz substrate, a ceramicsubstrate, or the like. Alternatively, a silicon substrate, a metalsubstrate, or a stainless steel substrate may be used if the surface ofthe substrate is formed with an insulating film. A plastic substratehaving a heat resistance against the process temperature of thisembodiment may also be used.

As shown in FIG. 7A, a base insulating film 101 is formed on thesubstrate 100 from an insulating film such as a silicon oxide film, asilicon nitride film, and a silicon oxynitride film. The base insulatingfilm 101 in this embodiment has a two-layer structure. However, it maybe a single layer of the insulating films given in the above, or alaminate consisting of more than two layers of the above insulatingfilms. The first layer 101 a of the base insulating film 101 is asilicon oxynitride film 101 a formed to a thickness of 30 to 100 nmusing as reaction gas SiH₄, NH₃, and N₂O. The second layer 101 b of thebase insulating film 101 is a silicon oxynitride film 101 b formed to athickness of 100 to 150 nm using as reaction gas SiH₄, and N₂O. The film101 b is layered on the film 101 a.

An amorphous semiconductor film is formed next on the base insulatingfilm 101. The thickness of the amorphous semiconductor film is 30 to 60nm. Though not limited, the material of the amorphous semiconductor filmis preferably silicon or a silicon germanium (Si_(x)Ge_(1−x); 0<x<1,typically x=0.001 to 0.05) alloy. In this embodiment, the amorphoussemiconductor film is formed by plasma CVD using SiH₄ gas.

The base insulating film and the amorphous semiconductor film can beformed by the same film formation method, and therefore the baseinsulating film and the amorphous semiconductor film may be formedsuccessively.

Next, the amorphous semiconductor film is subjected to a knowncrystallization treatment (such as laser crystallization, thermalcrystallization, or thermal crystallization using nickel or othercatalysts) to form a crystalline semiconductor film. The obtainedcrystalline semiconductor film is patterned into a desired shape. Inthis embodiment, a solution containing nickel is retained to the topface of the amorphous silicon film. The film is then subjected todehydrogenation (at 500° C. for an hour) followed by thermalcrystallization (at 550° C. for four hours) and laser annealingtreatment for improving crystallinity, whereby a crystalline siliconfilm is formed. The crystalline silicon film is patterned byphotolithography to form semiconductor layers 102 to 106.

After forming the semiconductor layers 102 to 106, the layers may bedoped with an impurity element imparting p-type conductivity in order tocontrol the threshold (Vth) of an n-channel TFT. Known impurity elementsthat can give a semiconductor the p type conductivity are elementsbelonging to Group 13 in the periodic table, such as boron (B), aluminum(Al), and gallium (Ga). In this embodiment, boron (B) is used in thedoping.

Besides, in the case where the crystalline semiconductor film ismanufactured by the laser crystallization method, a pulse oscillationtype or continuous emission type excimer laser, a YAG laser or a YVO₄laser may be used. In the case where those lasers are used, it isappropriate to use a method in which laser light radiated from a laseroscillator is condensed by an optical system into a linear beam, and isirradiated to the semiconductor film. The conditions of thecrystallization may be properly selected by an operator.

A gate insulating film 107 is then formed for covering the island-likesemiconductor layers 102 to 106. The gate insulating film 107 is formedof an insulating film containing silicon with a thickness of from 40 to150 nm by a plasma CVD method or a sputtering method. Of course, asingle layer or a lamination structure of an insulating film containingother silicon can be used for the gate insulating film.

When the silicon oxide film is used, it can be formed by a plasma CVDmethod in which TEOS (tetraethyl orthosilicate) and O₂ are mixed, with areaction pressure of 40 Pa, a substrate temperature of from 300 to 400°C., and discharged at a high frequency (13.56 MHz) power density of 0.5to 0.8 W/cm². The silicon oxide film thus manufactured can obtain goodcharacteristics as the gate insulating film by subsequent thermalannealing at 400 to 500° C.

Then, on the gate insulating film 107, a first conductive film (TaN) 108and a second conductive film (W) 109 are formed into lamination to havea film thickness of 20 to 100 nm and 100 to 400 nm, respectively Theconductive films forming a gate electrode may be formed of an elementselected from the group consisting of Ta, W, Ti, Mo, Al, and Cu, or analloy material or a compound material containing the above element asits main constituent. Further, a semiconductor film typified by apolycrystalline silicon film doped with an impurity element such asphosphorus may be used. Besides, any combination may be employed such asa combination in which the first conductive film is formed of tantalum(Ta) and the second conductive film is formed of W, a combination inwhich the first conductive film is formed of titanium nitride (TaN) andthe second conductive film is formed of Al, or a combination in whichthe first conductive film is formed of tantalum nitride (TaN) and thesecond conductive film is formed of Cu.

Next, masks 110 to 115 made from resist are formed using aphotolithography method, and a first etching process is performed inorder to form electrodes and wirings. In this embodiment, an ICP(inductively coupled plasma) etching method is used, a gas mixture ofCF₄, Cl₂ and O₂ is used as an etching gas, the gas flow rate is set to25/25/10 sccm, and a plasma is generated by applying a 500 W RF (13.56MHz) power to a coil shape electrode at 1 Pa, thereby performingetching. A 150 W RF (13.56 MHz) power is also applied to the substrateside (sample stage), effectively applying a negative self-bias voltage.The W film is etched with the first etching conditions, and a firstshape conductive film including the taper portion at the end portion isformed.

Thereafter, the first etching conditions are chanced into the secondetching conditions without removing the masks 110 to 115 made of resist,a gas mixture of CF₄ and Cl, is used as an etching gas, the gas flowrate is set to 30/30 sccm, and a plasma is generated by applying a 500 WRF (13.56 MHz) power to a coil shape electrode at 1 Pa, therebyperforming etching for about 30 seconds. A 20 W RF (13.56 MHz) power isalso applied to the substrate side (sample stage), effectively applyinga negative self-bias voltage. The W film and the TaN film are bothetched on the same order with the second etching conditions in which CF₄and Cl₂ are mixed. Note that, the etching time may be increased byapproximately 10 to 20% in order to perform etching without any residueon the gate insulating film.

In the first etching process, a first shape conductive layer is formedto have a tapered shape at the end portion due to the effect of the biasvoltage applied to the substrate side by adopting a suitable shape ofthe masks formed from resist. The angle of the tapered portions is setto 15 to 45°. Thus, first shape conductive layers 117 to 122 (firstconductive layers 117 a to 122 a and second conductive layers 117 b to122 b) are formed by the first etching process. Reference numeral 116denotes a gate insulating film, and regions of the gate insulating film,which are not covered by the first shape conductive layers 117 to 122,are made thinner by approximately 20 to 50 nm by etching.

Then, a first doping process is performed to add an impurity element forimparting n-type conductivity to the semiconductor layer withoutremoving the mask made of resist (FIG. 7B). Doping may be carried out b,an ion doping method or an ion implantation method. The condition of theion doping method is that a dosage is 1.5×10¹⁵/cm², and an accelerationvoltage is 60 to 100 keV. As the impurity element for imparting n-typeconductivity, an element belonging to group 15 of the periodic tabletypically phosphorus (P) or arsenic (As) is used. In this case, theconductive layers 117 to 121 become masks to the impurity elementimpairing n-type conductivity and the first concentration impurityregions 123 to 127 are formed in a self-aligning manner. The impurityelement imparting n-type conductivity in the concentration range of1×10²⁰ to 1×10²¹ atoms/cm³ is added to the first concentration impurityregions 123 to 127.

Thereafter, as shown in FIG. 7C, the second etching process is performedwithout removing the masks made of resist. Here, a gas mixture of CF₄,Cl₂ and O₂ is used as an etching gas, the gas flow rate is set to20/20/20 (sccm), and a plasma is generated by applying a 500 W RF (13.56MHz) power to a coil shape electrode at 1 Pa, thereby performingetching. A 20 W RF (13.56 MHz) power is also applied to the substrateside (sample stage), effectively applying a self-bias voltage which islower than that of the first etching process. According to the thirdetching condition. W film is etched. Thus, according to the thirdetching condition. W film is etched in a different direction to form theconductive films 129 to 134.

Etching reactions in etching the W film and the TaN film with a mixturegas of CF₄ and Cl₂ can be inferred from the kind of radicals or ionsgenerated and the vapor pressure of a reaction product. Comparing thevapor pressure among fluorides and chlorides of W and TaN, the vaporpressure of WF₆, which is a fluoride of W, is extremely high while therest of them, namely, WCl₅, TaF₅, and TaCl₅, have about the same levelof vapor pressure. Therefore the W film and the TaN film are etchedsimilarly with a mixture gas of CF₄ and Cl₂. If the mixture gas is addedwith an appropriate amount of O₂, CF₄ and O₂ reacts to change into COand F and a large amount of F radicals or F ions are generated. As aresult, the W film whose fluoride has high vapor pressure is etched atan increased etching rate. On the other hand, the etching rate of theTaN film does not increase much w hen F is increased. The surface of theTaN film is slightly oxidized by addition of O₂ to the mixture gasbecause TaN is more easily oxidized than W. The oxide of TaN does notreact with fluorine or chlorine, thereby further lowering the etchingrate of the TaN film. Accordingly, the etching rate of the W film can bedifferentiated from the etching rate of the TaN film so that the W filmis etched faster than the TaN film.

Next, a second doping treatment is conducted as shown in FIG. 8A withoutremoving the resist masks. In the second doping treatment, the layersare doped with an impurity element imparting n-type conductivity in adose smaller than in the first doping treatment and at a highacceleration voltage. The acceleration voltage is set to 70 to 120 keV,90 keV, in this embodiment. The dose is set to 1.5×10¹⁴ atoms/cm². Newimpurity regions are thus formed in the semiconductor layers inside thefirst concentration impurity regions formed in FIG. 8B. In the doping,the semiconductor layers under the second shape first conductive layers129 a to 13 a are also doped with the impurity element while using thesecond shape conductive layers 129 to 133 as masks.

Thus formed are third concentration impurity regions 140 to 144 andsecond concentration impurity regions 135 to 139. The thirdconcentration impurity regions 140 to 144 overlap the second shape firstconductive layers 129 a to 133 a, respectively. The second concentrationimpurity regions are placed between the first concentration impurityregions and the third concentration impurity regions (135 is between 145and 140, 136 is between 146 and 141, 137 is between 147 and 142. 138 isbetween 148 a and 143, and 139 is between 149 and 144).

Then the resist masks are removed. Thereafter, masks 150 and 151 arenewly formed from a resist to conduct a third etching treatment as shownin FIG. 8B. SF₆ and Cl₂ are used as the etching gas, the gas flow rateratio of them is set to 50/10 SCCM, and an RF (13.56 MHz) power of 500 Wis applied to a coiled electrode at a pressure of 1.3 Pa to generateplasma for 30 second etching. The substrate side (sample stage) receivesan RF (13.56 MHz) power of 10 W to apply a substantially negativeself-bias voltage. In this way, the TaN film is etched in a futurep-channel TFT and in a future pixel portion TFT under the above thirdetching conditions. Third shape conductive layers 152 to 155 are formedas a result.

In this specification, a future p-channel TFT refers to a TFT in themiddle of fabrication which is to serve as a p-channel TFT after thefabrication is completed. Similarly, a ‘future n-channel TFT’ refers toan unfinished TFT that is to function as an n-channel TFT after itscompletion.

The resist masks are removed and the gate insulating film is then etchedas shown in FIG. 8C. CHE₃ is used as the etching gas, the gas flow ratethereof is set to 35 SCCM, and an RF power of 800 W is applied togenerate plasma for the etching. Here, the second shape conductivelayers 129 and 131 and the third shape conductive layers 152 to 155serve as masks to cut off portions of the gate insulating film for eachTFT (157–162).

Next, as shown in FIG. 9A, the masks 164 to 166 are formed from resistand a third doping process is performed. In accordance with the thirddoping process, forth concentration impurity regions 167 to 172 areformed, in which the impurity element imparting conductivity opposite tothe above conductivity is added to the semiconductor layer that becomesan active layer of the p-channel TFT. The third shape conductive layers152 and 154 are used as masks to the impurity element, and the impurityelement that imparts the p-type conductivity is added, to thereby formthe forth concentration impurity regions in a self-aligning manner. Inthis embodiment, the fourth concentration impurity regions 167 to 172are formed by an ion doping method using diborane (B₂H₆). In the thirddoping process, the semiconductor layer forming the n-channel TFT iscovered with the masks 164 and 166 formed from resist. Althoughphosphorus is added to the forth concentration impurity regions 167 and172 at different concentrations in accordance with the first and seconddoping processes, the doping process is performed such that theconcentration of the impurity element imparting, p-type conductivity ishigher in any of the impurity regions. Thus, the impurity regionsfunction as the source region and the drain region of the p-channel TFTso that no problem occurs.

In accordance with these processes, the impurity regions are formed onthe respective semiconductor layers. In this embodiment, all theimpurity regions are formed in a self-aligning manner, with theconductive layer as a mask. The third shape conductive layers 129, 130,152, and 153 which overlap the semiconductor layers function as gateelectrodes. Besides, the conductive layer 155 functions as source wiringand the conductive layer 154 functions as the capacitor wiring which isone of storage capacity.

Subsequently, the masks 164 and 166 consisting of resist are removed,and a first interlayer insulating film 173 covering the whole surface isformed. This first interlayer insulating film 173 is formed of aninsulating film containing silicon with a thickness of 100 to 200 nm bya plasma CVD method or a sputtering method. In this embodiment, asilicon oxynitride film with a film thickness of 150 nm is formed by aplasma CVD method. Of course, the first interlayer insulating film 173is not particularly limited to the silicon oxynitride film, and otherinsulating films containing silicon may be formed into a single layer ora lamination structure.

Then, as shown in FIG. 9B, a step of activating the impurity elementsadded in the respective semiconductor layers is performed. This step iscarried out by thermal annealing using an annealing furnace. The thermalannealing may be performed in a nitrogen atmosphere having an oxygenconcentration of 100 ppm or less, preferably 0.1 ppm or less and at 400to 700° C., typically 500 to 550° C. Note that, in addition to thethermal annealing method, a laser annealing method, or a rapid thermalannealing method (RTA method) can be applied thereto.

Note that, in this embodiment, at the same time as the above activationprocess, nickel used as the catalyst for crystallization is gettered tothe regions (145 to 149, 167, 170) containing phosphorus at a highconcentration. As a result, mainly nickel concentration of thesemiconductor layer which becomes a channel formation region is lowered.The TFT having a channel formation region thus formed is decreased inof: current value, and has high electric field mobility because of goodcrystallinity, thereby attaining satisfactory characteristics.

Next, a second interlayer insulating film 174 made of an organicinsulating material is formed on the first interlayer insulating film173. Then, patterning is performed for forming a contact hole reachingthe source wiring 155 and contact holes reaching the respective impurityregions 145, 147, 148 a 167 and 170.

Then, in a driver circuit 406, wirings 175 to 180 electrically connectedto the first concentration impurity region and the fourth concentrationimpurity region, respectively, are formed. Note that, these wirings areformed by patterning a lamination film of a Ti film with a filmthickness of 50 to 250 nm and an alloy film (alloy film of Al and Ti)with a film thickness of 300 to 500 nm.

Besides, in the pixel portion 1407, a pixel electrode 183, a gatewiring, 182 and a connecting electrode 181 are formed (FIG. 9C). Thesource wiring 155 is electrically connected with the pixel TFT 1404 bythe connecting electrode 181. Further, the gate wiring 182 iselectrically connected with a third shape conductive layer 153 (a gateelectrode of the pixel TFT. Furthermore, the pixel electrode 183 iselectrically connected with the drain region of the pixel TFT and withthe semiconductor layer functioning as one of electrodes forming astorage capacity. Preferably, as the pixel electrode 183, the filmcomposed of Al or Ag as its main constituent, or a lamination film ofthe films, which is superior in reflection.

In the manner as described above, the driver circuit 1406 including ann-channel TFT 1401, a p-channel TFT 1402, and an n-channel TFT 1403, andthe pixel portion 1407 including the pixel TFT 1404 and a storagecapacitor 1405 can be formed on the same substrate. In thisspecification, such a substrate is called an active matrix substrate forconvenience.

The n-channel TFT 1401 of the driver circuit 1406 includes a channelformation region 184, the third concentration impurity region 140 (GOLDregion) overlapping with the third shape conductive layer 129 formingthe gale electrode, the second concentration impurity region 135 (LDDregion) formed outside the gate electrode, and the first concentrationimpurity region 145 functioning as a source region or a drain region.The p-channel TFT 1402 includes a channel formation region 185, forthconcentration impurity regions 168 and 169, which are formed outside thegate electrode, and a forth concentration impurity region 167functioning as a source region or a drain region. The n-channel TFT 1403includes a channel formation region 186, the third concentrationimpurity region 142 (GOLD region) overlapping the third shape conductivelayer 131 forming the gate electrode, the second concentration impurityregion 137 (LDD region) formed outside the gate electrode, and the firstconcentration impurity region 147 functioning as a source region or adrain region.

The pixel TFT 1404 of the pixel portion includes a channel formationregion 187, the third concentration impurity region 143 (LDD region)formed outside the gate electrode, the second concentration impurityregion 138 (LDD region), and the first concentration impurity region 148a functioning as a source region or a drain region. Besides, impurityelements imparting p-type conductivity are added at the sameconcentration as the forth concentration impurity region to therespective semiconductor layers 170 to 172 functioning as one ofelectrodes of the storage capacitor 1405. The storage capacitor 1405 isformed by the capacitor wiring 154 and the semiconductor layers 170 to172 with the insulating film (the same film as the gate insulting film)as a dielectric.

In this embodiment, an optimal structure is chosen for the respectiveTFTs constituting the circuits in accordance with circuit specificationsrequired for the pixel portion and the driver circuit, so that theoperation performance and the reliability of the semiconductor deviceare improved. Specifically, the LDD structure or the GOLD structure ischosen for an n-channel TFT according to the circuit specification. Thusa TFT structure giving priority to high-speed operation or hot carriercountermeasure and a TFT structure giving priority to low OFF currentoperation can be formed on the same substrate.

For instance, in the case of an active matrix liquid crystal displaydevice, the n-channel TFTs 1401 and 1403 are suitable for drivercircuits for which high-speed operation is more important, such as ashift register, a frequency dividing circuit a signal dividing circuit,a level shifter, and a buffer. In other words, a TFT obtains a structurethat places stress on hot carrier countermeasures by having a GOLDregion.

The pixel TFT 1404 is an n-channel TFT having a structure that givespriority to low OFF current operation. This TFT is therefore applicableto a sampling circuit other than the pixel portion. The TFT has no GOLDregion that can increase the OFF current value but has an LDD region andan offset region to obtain low OFF current operation. In addition, ithas been confirmed that the first concentration impurity region 148 b isvery effective in reducing the OFF current value.

FIG. 10 shows the top view of a pixel portion on an active matrixsubstrate fabricated in accordance with this embodiment. In FIG. 10,components corresponding to those in FIGS. 7A to 9C are denoted by thesame reference symbols. The sectional view taken along the dot-dashedline A–A′ in FIG. 10 corresponds to the one taken along the dot-dashedline A–A′ in FIG. 9. The sectional view taken along the dot-dashed lineB–B′ in FIG. 10 corresponds to the one taken along the dot-dashed lineB–B′ in FIG. 9.

As illustrated in the drawings, the active matrix substrate having thepixel structure of this embodiment is characterized in that, the gateelectrode 153 of the pixel TFT and the gate line 182 are formed indifferent layers so that the semiconductor layer is shielded from lightby the gate line 182.

According to the pixel structure of this embodiment, the pixelelectrodes are arranged so that edges of the pixel electrodes overlapthe source wiring line in order to shield gaps between the pixelelectrodes against light without using a black matrix.

The surfaces of the pixel electrodes according to this embodiment aredesirably made uneven by a known method, e.g., the sand blast method oretching, in order to increase the white light level by preventingregular reflection and scattering the reflected light.

The pixel structure described above makes it possible to arrange pixelelectrodes having a larger area to improve the aperture ratio.

The manufacture process shown in this embodiment requires only six photomasks to fabricate an active matrix substrate (namely, a semiconductorlayer pattern mask, a first wiring line pattern mask (including the gateelectrode 153 of the pixel TFT, the capacitor wiring line 154, and thesource line 155), a pattern mask for forming conductive layers of thep-channel TFT and of the pixel portion TFT, a pattern mask for formingthe source region and the drain region of the p-channel TFT, a patternmask for forming contact holes, and a second wiring pattern mask(including the pixel electrode 183, the connector electrode 181, and thegate line 182)). Therefore this embodiment can contribute to cutting theprocess and the manufacture cost and improving the yield.

FIG. 11 shows a sectional view of an active matrix substrate suitablefor a transmissive liquid crystal display device. The manufactureprocess of this substrate is the same as the substrate for the abovereflective liquid crystal display device up through the step of forminga second interlayer insulating film. On the second interlayer insulatingfilm, a transparent conductive film is formed and then patterned to forma transparent conductive layer 191. The transparent conductive film maybe formed of a compound of indium oxide and tin oxide, or a compound ofindium oxide and zinc oxide.

In the driver circuit 1406, wiring lines 175 to 180 electricallyconnected to the first concentration impurity regions or the fourthconcentration impurity regions are formed. The wiring lines are formedby patterning a laminate of a Ti film with a thickness of 50 to 250 nmand an alloy film (of Al and Ti) with a thickness of 300 to 500 nm. Onthe other hand, a pixel electrode 191, a gate line 182, and connectorelectrodes 192 and 193 are formed in the pixel portion 1407. Theconnector electrodes 192 and 193 are formed so as to overlap the pixelelectrode 191. In this way, the active matrix substrate suitable for thetransmissive liquid crystal display can be manufactured when one moremask is used.

TFTs according to this embodiment have displayed excellentcharacteristics. Of those, the pixel TFT is picked to show its TFTcharacteristic (the V-I characteristic), which is graphed in FIG. 37.The gate leak is also shown in the graph and it is sufficiently low. Thepixel TFT structure of the present invention is particularly capable oflowering OFF current, and also rates well in terms of mobility. OFFcurrent is a drain current flowing when a TFT is in an OFF state.

While FIG. 37 is a V-I characteristic graph of Samples 1 through 8. FIG.3S shows the TFT characteristic of Sample 3.

Having the structure of the present invention. Sample 3 shows as smallthreshold (Vth) as 0.263 V, which is desirable (Vth is the voltage atthe rising point in the V-I characteristic graph). The smaller thedifference becomes, the more the short channel effect is contained.Sample 3 has a mobility of 119.2 cm²/Vs, meaning it is also excellent inmobility (μ_(FE)) that is a parameter indicating easiness for carriersto move. The S value (subthreshold coefficient), which is the reciprocalof the maximum inclination in the rising part of the I-V curve, is 0.196V/decade in Sample 3. When VD=5V, OFF current (I_(OFF2)) is 0.39 pA,whereas ON current (I_(ON2)) is 70 μA. ON current is a drain currentflowing when a TFT is in an ON state. Shift-1 denotes the voltage at therising of the I-V curve.

As described above, employing the present invention results in asemiconductor device having excellent characteristics.

Embodiment 2

FIG. 39 shows a p-channel TFT 2100 and an n-channel TFT 2200 of aninverter circuit manufactured in accordance with the present invention.These TFTs are formed on a base insulating film 2002 that is formed on asubstrate 2001.

The p-channel TFT 2100 has a semiconductor layer 2003 a gate insulatingfilm 2021, and a gate electrode that is composed of a first conductivelayer 2005 a and a second conductive layer 2005 b. The semiconductorlayer 2003 includes a channel formation region 2012, a source region2013 connected with a source electrode 2009, a drain region 2014connected with a drain electrode 2018, and an LDD region 2015 sandwichedbetween the drain region and the channel formation region. Referencesymbols 2007 and 2008 denote first and second interlayer insulatingfilms, respectively.

In the gate electrode, the end of the first conductive film 2005 a andthe end of the second conductive film 2005 b almost coincide with eachother on the source region side whereas the end of the first conductivefilm 2005 a on the drain region side is extended outward. This structureis obtained by forming a resist mask in the third etching treatmentshown in FIG. 8B so as to cover only one side of the gate electrode.

Thereafter, the semiconductor layer 2003 in the p-channel TFT is dopedwith a p type impurity element by ion doping or the like to form animpurity region in the semiconductor layer. The LDD region 2015 can beformed by using the first conductive film 2005 a as a mask. In iondoping, it is possible to form the LDD region as well as the sourceregion and the drain region in a single doping treatment by controllingthe acceleration voltage. Instead, the doping treatment may be conductedtwice while optimizing the acceleration voltage, so that formation ofthe LDD region is separated from formation of the source region and thedrain region.

On the other hand, the n-channel TFT 2200 has a semiconductor layer2004, a gate insulating film 2022, and a gate electrode that is composedof a first conductive film 2006 a and a second conductive film 2006 b.The semiconductor layer 2004 includes a channel formation region 2016, asource region 2017 connected with a source electrode 2010, a drainregion 2018, and LDD regions 2019 and 2020.

Similar to the p-channel TFT, the end of the first conductive film 2006a and the end of the second conductive film 2006 b in the gate electrodeof the n-channel TFT 2200 almost coincide with each other on the sourceregion side whereas the end of the first conductive film 2006 a on thedrain region side is extended outward. The LDD region 2019 on the sourceregion side is an LDD region that does not overlap with the gateelectrode, whereas the LDD region 2020 on the drain region side overlapsthe gate electrode.

The LDD regions overlapping the gate electrodes are formed on the drainside in the p-channel TFT and the n-channel TFT as described above. Thiseases the electric field intensity near a drain and prevents degradationof a TFT due to hot carriers. The preventive effect is needed also in ap-channel TFT especially when the channel length is in submicron level.

However, an LDD region overlapping a gate electrode increases aparasitic capacitance applied to the gate electrode and hence is notalways be provided on the source side where there is no need to easeelectric field.

According to the present invention, it is possible to form the LDDregion only on the drain side as shown in FIG. 39. Furthermore, theinvention can readily be applied to the case of employing a minutedesign rule, because the source region, the drain region, and the LDDregion are all formed in a self-aligning manner.

The TFT structure according to this embodiment can most effectively beapplied to a TFT in which the position of a drain region is determinedin advance as in an inverter circuit. The TFT structure of thisembodiment can be combined freely with the manufacture process ofEmbodiment 1 by merely changing the resist mask pattern.

Embodiment 3

In the p-channel TFT and the n-channel TFT of the inverter circuit shownin Embodiment 2, degradation due to hot carriers is not noticeable whenthe driving voltage is 10 V or lower. Then the LDD region overlapping agate electrode may not necessarily be formed. In this case, thep-channel TFT has the same structure as the p-channel TFT 402 shown inFIG. 11 while the n-channel TFT has the same structure as the n-channelTFT 404 shown in FIG. 11 and takes a single gate structure.

Embodiment 4

If the channel length is set to 0.6 μm or less in the active matrixsubstrate described in Embodiment 1, it is desirable to form the LDDregion overlapping a gate electrode also in the p-channel TFT. In thiscase, the LDD region is formed in the same way as the LDD region of then-channel TFT 1401 is formed to obtain the same structure, but is dopedwith a p type impurity element. The LDD region is provided only on thedrain side as shown in Embodiment 2, if the direction of a source and adrain is already determined as in shift register circuits and buffercircuits.

Embodiment 5

This embodiment gives a description of a case of manufacturing TFTs in adifferent step other than Embodiment 1 with reference to FIGS. 12A and12B. Embodiment 5 is merely different from Embodiment 1 in some stepsand the rest is the same. Therefore the same reference symbols are usedin the identical steps. The impurity elements used in doping are alsothe same.

First, the first etching treatment and the first doping treatment areconducted in accordance with the manufacture process shown in Embodiment1 to reach the state illustrated in FIG. 7B.

Thereafter, etching is made under the second etching conditions withoutremoving the resist masks 110 to 115. According to the second etchingconditions. CF₄ and Cl₂ are used as the etching gas, the gas flow rateratio of them is set to 30/30 SCCM, and an RF (13.56 MHz) power of 500 Wis applied to a coiled electrode at a pressure of 1 Pa to generateplasma for 30 second etching. The substrate side (sample stage) alsoreceives an RF (13.56 MHz) power of 20 W to apply a substantiallynegative self-bias voltage. The conductive film (A), i.e., the TaN film,and the conductive film (B), i.e., the W film are etched to the sameextent under the second etching conditions using a mixture of CF₄ andCl₂. As a result, a first shape gate electrode and wiring lines 217 to223 are formed. The gate electrode is composed of first shape firstconductive films 217 a to 223 a and first shape second conductive films217 b to 223 b.

A second doping treatment is conducted without removing the resist masks110 to 115. The semiconductor layers 102 to 106 are doped with animpurity element imparting n-type conductivity (hereinafter referred toas n type impurity element). The doping treatment is achieved by iondoping or ion implantation. The n type impurity element to be used is anelement belonging to Group 15 in the periodic table, typically,phosphorus (P) or arsenic (As). In this treatment, the first shape gateelectrode and capacitance wiring lines 217 to 221 serve as masks to formfirst concentration impurity regions 224 a to 224 e in a self-aligningmanner (FIG. 12A).

Still keeping the resist masks 110 to 115 in place, a third etchingtreatment is conducted. CF₄, Cl₂, and O₂ are used as the etching gas,the gas flow rate ratio of them is set to 20/20/20 SCCM, and an RF(13.56 MHz) power of 500 W is applied to a coiled electrode at apressure of 1.0 Pa to generate plasma for the etching. The substrateside (sample stage) receives an RF (13.56 MHz) power of 20 W for 80second etching treatment. As a result, a second shape gate electrode andwiring lines 225 to 231 are formed. The gate electrode is composed ofsecond shape first conductive films 225 a to 231 a and second shapesecond conductive films 225 b to 231 b.

Then a third doping treatment is conducted without removing the resistmasks 110 to 115. In the third doping treatment, the semiconductorlayers under the second shape first conductive films (TaN films) arealso doped with an n type impurity element while using the second shapeconductive layer and capacitance wiring lines 225 to 229 as masks.Formed as a result of this treatment between the first concentrationimpurity regions and the channel formation regions are secondconcentration impurity regions 232 a to 232 e each containing the n typeimpurity element in a concentration of 1×10¹⁸ to 1×10¹⁹ atoms/cm³. Thefirst concentration impurity regions 224 a to 224 e each contain the ntype impurity element in a concentration of 1×10²⁰ to 1×10²¹ atoms/cm³(FIG. 12B).

Next, the resist masks 110 to 115 are removed and masks 233 to 234 forcovering a future n-channel TFT and a future pixel TFT are formed from aresist to conduct a fourth doping treatment. The semiconductor layersare doped with a p type impurity element in a future first p-channel TFTand in a future second p-channel TFT while using the second shapeconductive layers 226 and 227 and the capacitance wiring line 229 asmasks. Fourth concentration impurity regions 235 a to 235 c and fifthconcentration impurity regions 235 d to 235 f are thus formed in aself-aligning manner. In this embodiment, p type impurity regions areformed by ion doping using diborane (B₂H₆). The fourth concentrationimpurity regions (P⁺) 235 a to 235 c each contain the p type impurityelement in a concentration of 2×10²⁰ to 1×10²¹ atoms/cm³. The fifthconcentration impurity regions 235 d to 235 f each contain the p typeimpurity element in a concentration of 2×10¹⁷ to 1×10¹⁹ atoms/cm³.Although the semiconductor layers of the p-channel TFTs have previouslybeen doped with the n type impurity element, the layers do not have aproblem to function as source regions and drain regions of the futurep-channel TFTs if they are doped with the p type impurity element in thefourth doping treatment in a concentration higher than the concentrationof the n type impurity element (FIG. 13A).

Resist masks 236 and 237 are then used to cover the n-channel TFT andthe first p-channel TFT of the driver circuit to conduct a fourthetching treatment. Cl₂ is used as the etching gas, the gas flow ratethereof is set to 80 SCCM, and an RF power of 350 W is applied to acoiled electrode at a pressure of 1.2 Pa to generate plasma for 30second etching. The substrate (sample stage) side receives an RF (13.56MHz) power of 50 W to apply a substantially negative self-bias voltage.Thus formed in the second p-channel TFT of the driver circuit and in thepixel TFT of the pixel portion third shape conductive layers (composedof third shape first conductive films 238 a and 239 a and third shapesecond conductive films 238 b and 239 b) 238 and 239, a capacitancewiring line 240, and wiring lines 241 and 242 (FIG. 13B). Through theabove treatment, the exposed portions of the gate insulating film onwhich the third shape conductive layers are not formed have obtained athickness of about 30 nm in the pixel portion and a thickness of about40 nm in the driver circuit.

The impurity regions are formed in the respective semiconductor layersthrough the above steps. For the subsequent steps to complete the activematrix substrate, see the step of forming an inorganic interlayerinsulating film and the following steps thereof disclosed in Embodiment1.

This embodiment can readily be carried out by manufacturing a TFT inaccordance with the manufacture process disclosed in Embodiment 1.Although this embodiment describes only the structure of the pixel TFTand the control circuit, other circuits can also be formed on the samesubstrate when following the manufacture process of Embodiment 1.Examples of the other circuits include a signal dividing circuit, afrequency dividing circuit, a D/A converter circuit, an operationamplifier circuit, a γ correction circuit, and a signal processingcircuit (also called a logic circuit) such as a memory circuit and amicroprocessor circuit.

Embodiment 6

This embodiment gives a description of a case of manufacturing TFTs in adifferent step other than Embodiment 1 with reference to FIGS. 14A to14C. Embodiment 6 is merely different from Embodiment 1 in some stepsand the rest is the same. Therefore the same reference symbols are usedin the identical steps. The impurity elements used in doping are alsothe same.

First, the first etching treatment and the first doping treatment areconducted in accordance with the manufacture process shown in Embodiment1 to reach the state illustrated in FIG. 7B. Thereafter, a secondetching treatment is conducted. CF₄, Cl₂ and O₂ are used as the etchinggas, the gas flow rate ratio of them is set to 20/20/20 SCCM, and an RF(13.56 MHz) power of 500 W is applied to a coiled electrode at apressure of 1.0 Pa to generate plasma for 60 second etching. Thesubstrate (sample stage) side also receives an RF (13.56 MHz) power of20 W to apply a substantially negative self-bias voltage. As a result ofthe second etching treatment, second shape conductive layers 301 to 304and wiring lines 305 to 307 are formed.

Next, the semiconductor layers are doped with an n type impurity elementthrough the second shape first conductive films in a self-aligningmanner while using the second shape second conductive films as masks.Formed as a result of this treatment between the channel formationregions and first concentration impurity regions 308 a to 308 e aresecond concentration impurity regions 308 f to 308 j each containing then type impurity element in a concentration of 1×10¹⁸ to 1×10¹⁹atoms/cm³. At this point, the first concentration impurity regions 308 ato 308 e each contain the n type impurity element in a concentration of1×10²⁰ to 1×10²¹ atoms/cm³.

Then the resist masks 110 to 115 are removed. Thereafter, masks 309 and310 for covering the n-channel TFT and the pixel TFT are newly formedfrom a resist to conduct a third doping treatment. Through the thirddoping treatment, the semiconductor layers in the p-channel TFTs aredoped with a p type impurity element in a self-aligning manner whileusing the second shape conductive layers as masks. Fourth concentrationimpurity regions 311 a to 311 c and fifth concentration impurity regions311 d to 311 f are thus formed (FIG. 14B).

The resist masks 309 and 310 are removed, and masks 312 and 313 arenewly formed from a resist to cover the n-channel TFT and the secondp-channel TFT. Cl₂ is used as the etching gas, the gas flow rate thereofis set to 80 SCCM, and an RF (13.56 MHz) power of 500 W is applied to acoiled electrode at a pressure of 1.2 Pa to generate plasma for 40second etching. The substrate (sample stage) side receives an RF (13.56MHz) power of 10 W to apply a substantially negative self-bias voltage.Thus formed in the first p-channel TFT and in the pixel TFT are thirdshape conductive layers (composed of third shape first conductive films314 a and 315 a and third shade second conductive films 314 b and 315 b)314 and 315, and wiring lines 316 to 318 (FIG. 14C).

Through the third etching treatment, offset regions 311 g and 311 h areformed in the semiconductor layers of the first p-channel TFT and of thepixel TFT, respectively. An offset region in this specification refersto a semiconductor layer having the same composition as a channelformation region (meaning the region contains the same impurity elementas the channel formation region), and the region does not overlap a gateelectrode. The offset regions 311 g and 311 h function as simpleresistors and are very effective in reducing the OFF current value.

For the subsequent steps to complete the active matrix substrate, seethe step of forming an inorganic interlayer insulating film and thefollowing steps thereof disclosed in Embodiment 1.

This embodiment can readily be carried out by manufacturing a TFT inaccordance with the manufacture process disclosed in Embodiment 1.Although this embodiment describes only the structure of the pixel TFTand the control circuit, other circuits can also be formed on the samesubstrate when following the manufacture process of Embodiment 1.Examples of the other circuits include a signal dividing circuit, afrequency dividing circuit, a D/A converter circuit, an operationamplifier circuit, a γ correction circuit, and a signal processingcircuit (also called a logic circuit) such as a memory circuit and amicroprocessor circuit.

Embodiment 7

This embodiment gives a description of a case of manufacturing TFTs in adifferent step other than Embodiment 1 with reference to FIGS. 15A to15C. Embodiment 7 is merely different from Embodiment 1 in some stepsand the rest is the same. Therefore the same reference symbols are usedin the identical steps.

First, the first etching treatment and the first doping treatment areconducted in accordance with the manufacture process shown in Embodiment1 to reach the state illustrated in FIG. 7B. Thereafter, a secondetching treatment is conducted. In the second etching treatment. CF₄,Cl₂ and O₂ are used as the etching gas, the gas flow rate ratio of themis set to 20/20/20 SCCM, and an RF (13.56 MHz) power of 500 W is appliedto a coiled electrode at a pressure of 1.0 Pa to generate plasma for 80second etching. The substrate (sample stage) side receives an RF (13.56MHz) power of 20 W to apply a substantially negative self-bias voltage.Thus second shape conductive layers and wiring lines are formed.

Next, the n-channel TFT and the pixel TFT are covered with resist masks401 and 402, respectively, to conduct a second doping treatment. Throughthe second doping treatment, the semiconductor layers in the p-channelTFTs are doped with a p type impurity element. The semiconductor layersare doped with the p type impurity element through the second shapefirst conductive films in a self-aligning manner while using the secondshape second conductive films as masks. As a result, fourthconcentration impurity regions 403 a to 403 c and fifth concentrationimpurity regions 403 d to 403 f are formed (FIG. 15A).

The n-channel TFT and the second p-channel TFT are then covered withresist masks 404 and 405, respectively, to conduct a third etchingtreatment. Cl₂ is used as the etching gas, the gas flow rate thereof isset to 80 SCCM, and an RF (13.56 MHz) power of 500 W is applied to acoiled electrode at a pressure of 1.2 Pa to generate plasma for 40second etching. The substrate (sample stage) side receives an RF (13.56MHz) power of 20 W to apply a substantially negative self-bias voltage.Thus third shape conductive layers 406 and 407 and wiring lines 408 to410 are formed (FIG. 15B).

Next, the resist masks 404 and 405 are removed to conduct a third dopingtreatment. In the third doping treatment, the semiconductor layers aredoped with an n type impurity element to form impurity regions 411 a and411 b. The semiconductor layers in the p-channel TFTs do not have aproblem to function as source regions and drain regions of the p-channelTFTs because the regions have already been doped with the p typeimpurity element in a concentration higher than the concentration of then type impurity element (FIG. 15C).

After finishing the steps described above, the active matrix substrateis completed in accordance with the step of forming an inorganicinterlayer insulating film and the following steps thereof disclosed inEmbodiment 1.

This embodiment can readily be carried out by manufacturing a TFT inaccordance with the manufacture process disclosed in Embodiment 1.Although this embodiment describes only the structure of the pixel TFTand the control circuit, other circuits can also be formed on the samesubstrate when following the manufacture process of Embodiment 1.Examples of the other circuits include a signal dividing circuit, afrequency dividing circuit, a D/A convener circuit, an operationamplifier circuit, a y correction circuit, and a signal processingcircuit (also called a logic circuit) such as a memory circuit and amicroprocessor circuit.

Embodiment 8

This embodiment gives a description of a case of manufacturing TFTs in adifferent step other than Embodiment 1 with reference to FIGS. 16A to16C. Embodiment 8 is merely different from Embodiment 1 in some stepsand the rest is the same. Therefore the same reference symbols are usedin the identical steps.

First, the second etching treatment and the second doping treatment areconducted in accordance with the manufacture process shown in Embodiment1 to reach the state illustrated in FIG. 7C.

Next, a mask 501 is formed from a resist to cover the n-channel TFT anda third etching treatment is conducted. In the third etching treatment.Cl₂ is used as the etching gas, the gas flow rate thereof is set to 80SCCM, and an RF (13.56 MHz) power of 350 W is applied to a coiledelectrode at a pressure of 1.2 Pa to generate plasma for 40 secondetching. The substrate (sample stage) side receives an RF (13.56 MHz)power of 50 W to apply a substantially negative self-bias voltage. Thusthird shape conductive layers and wiring lines 502 to 507 are formed(FIG. 16B).

After the resist mask is removed, the gate insulating film is etched.CHF₃ is used as the etching gas, the gas flow rate thereof is set to 35SCCM, and an RF (13.56 MHz) power of 800 W is applied to generate plasmafor the etching. The substrate (sample stage) side receives an RF (13.56MHz) power of 20 W to apply a substantially negative self-bias voltage.Here, the second shape gate electrode serves as a mask for the n-channelTFT whereas the third shape conductive layers and the capacitance wiringlines serve as masks for the other TFTs, and portions of the gateinsulating film are cut off for each TFT to form gate insulating films508 to 514 (FIG. 16C).

Then masks 515 and 516 are newly formed from a resist to conduct a thirddoping treatment. Through the third doping treatment, the semiconductorlayers in the p-channel TFTs are doped with a p type impurity elementwhile using the third shape gate electrode and capacitance wiring linesas masks. Fourth concentration impurity regions 517 a to 517 c and fifthconcentration impurity regions 517 d to 517 f are thus formed in aself-aligning manner (FIG. 17).

After finishing the steps described above, the active matrix substrateis completed in accordance with the step of forming an inorganicinterlayer insulating film and the following steps thereof disclosed inEmbodiment 1.

This embodiment can readily be carried out by manufacturing a TFT inaccordance with the manufacture process disclosed in Embodiment 1.Although this embodiment describes only the structure of the pixel TFTand the control circuit, other circuits can also be formed on the samesubstrate when following the manufacture process of Embodiment 1.Examples of the other circuits include a signal dividing circuit, afrequency dividing circuit, a D/A converter circuit, an operationamplifier circuit, a γ correction circuit, and a signal processingcircuit (also called a logic circuit) such as a memory circuit and amicroprocessor circuit.

Embodiment 9

This embodiment gives a description of a case of manufacturing TFTs in adifferent step other than Embodiment 1 with reference to FIGS. 18A to18C. Embodiment 9 is merely different from Embodiment 1 in some stepsand the rest is the same. Therefore the same reference symbols are usedin the identical steps.

First, the second etching treatment and the second doping treatment areconducted in accordance with the manufacture process shown in Embodiment1 to reach the state of FIG. 7C where the second shape conductive layersand wiring lines are formed.

Next, the n-channel TFT is covered with a resist mask 601 to conduct athird etching treatment. Cl₂ is used as the etching gas, the gas flowrate thereof is set to 80 SCCM, and an RF (13.56 MHz) power of 350 W isapplied to a coiled electrode at a pressure of 1.2 Pa to generate plasmafor 40 second etching. The substrate (sample stage) side receives an RF(13.56 MHz) power of 50 W to apply a substantially negative self-biasvoltage. Thus third shape conductive layers and wiring lines 602 to 607are formed (FIG. 18B).

The resist mask 601 is then removed and masks 608 and 609 are newlyformed from a resist to cover the n-channel TFT and the pixel TFT,respectively. A third doping treatment is conducted and thesemiconductor layers are doped with a p type impurity element to formfourth concentration p type impurity regions 610 a to 610 c and fifthconcentration impurity regions 610 d to 610 f (FIG. 18C).

After finishing the steps described above, the active matrix substrateis completed in accordance with the step of forming an inorganicinterlayer insulating film and the following steps thereof disclosed inEmbodiment 1.

This embodiment can readily be carried out by manufacturing a TFT inaccordance with the manufacture process disclosed in Embodiment 1.Although this embodiment describes only the structure of the pixel TFTand the control circuit, other circuits can also be formed on the samesubstrate when following the manufacture process of Embodiment 1.Examples of the other circuits include a signal dividing circuit, afrequency dividing circuit, a D/A converter circuit, an operationamplifier circuit, a γ correction circuit, and a signal processingcircuit (also called a logic circuit) such as a memory circuit and amicroprocessor circuit.

Embodiment 10

This embodiment gives a description of a case of manufacturing TFTs in adifferent step other than Embodiment 1 with reference to FIGS. 19A to19C. Embodiment 10 is merely different from Embodiment 1 in some stepsand the rest is the same. Therefore the same reference symbols are usedin the identical steps.

First, the first etching treatment and the first doping treatment areconducted in accordance with the manufacture process shown in Embodiment1 to reach the state illustrated in FIG. 7B. A second etching treatmentis conducted next. First etching conditions for the second etchingtreatment are as follows: CF₄ and Cl₂ are used as the etching gas, thegas flow rate ratio of them is set to 30/30 SCCM, and an RF (13.56 MHz)power of 500 W is applied to a coiled electrode at a pressure of 1.0 Pato generate plasma for 30 second etching. The substrate (sample stage)side receives an RF (13.56 MHz) power of 20 W to apply a substantiallynegative self-bias voltage (FIG. 19B). The treatment is then followed byetching under second etching conditions: CF₄, Cl₂ and O₂ are used as theetching gas, the gas flow rate ratio of them is set to 20/20/20 SCCM.and an RF (13.56 MHz) power of 500 W is applied to a coiled electrode ata pressure of 1.0 Pa to generate plasma for 60 second etching. Thesubstrate (sample stage) side receives an RF (13.56 MHz) power of 20 Wto apply a substantially negative self-bias voltage. Thus second shapeconductive layers and wiring lines 701 to 707 are formed (FIG. 19C).

A second doping treatment is conducted next. The semiconductor layersare doped with an n type impurity element while using the second shapegate electrode and capacitance wiring lines as masks. As a result,second concentration impurity regions 708 a to 708 e each containing then type impurity element in a concentration of 1×10¹⁸ to 1×10¹⁹ atoms/cm³are formed in a self-aligning manner. At this point, the firstconcentration impurity regions each contain the n type impurity elementin a concentration of 1×10²⁰ to 1×10²¹ atoms/cm³ (FIG. 20A).

In this embodiment, the second etching treatment is divided into twostages to etch the conductive films. The etching treatment under thefirst conditions removes the edges of the first conductive films. Thisresults in formation of L_(ov) regions in which the gate electrodeoverlaps the second concentration impurity regions with the gateinsulating film interposed therebetween and L_(off) regions 719 in whichthe gate electrode does not overlap the second concentration impurityregions.

Next, a mask 709 is formed from a resist and covers the n-channel TFT toconduct a third etching treatment. In the third etching treatment. Cl₂is used as the etching gas, the gas flour rate thereof is set to 80SCCM, and an RF (13.56 MHz) power of 350 W is applied to a coiledelectrode at a pressure of 1.2 Pa to generate plasma for 40 secondetching. The substrate (sample stage) side receives an RF (13.56 MHz)power of 50 W to apply a substantially negative self-bias voltage. Thusthird shape conductive layers and wiring lines 710 to 715 are formed(FIG. 20B).

Masks 716 and 717 are newly formed from a resist to cover the n-channelTFT and the pixel TFT, respectively, in preparation for a third dopingtreatment. Through the third doping treatment, the semiconductor layersin the p-channel TFTs are doped with a p type impurity element whileusing the third shape conductive layers and the capacitance wiring linesas masks. Fourth concentration impurity regions 718 a to 718 c and fifthconcentration impurity regions 718 d to 718 f are thus formed in aself-aligning manner (FIG. 20C).

After finishing the steps described above, the active matrix substrateis completed in accordance with the step of forming an inorganicinterlayer insulating film and the following steps thereof disclosed inEmbodiment 1.

This embodiment can readily be carried out by manufacturing a TFT inaccordance with the manufacture process disclosed in Embodiment 1.Although this embodiment describes only the structure of the pixel TFTand the control circuit, other circuits can also be formed on the samesubstrate when following the manufacture process of Embodiment 1.Examples of the other circuits include a signal dividing circuit, afrequency dividing circuit, a D/A converter circuit, an operationamplifier circuit, a γ correction circuit, and a signal processingcircuit (also called a logic circuit) such as a memory circuit and amicroprocessor circuit.

Embodiment 11

This embodiment gives a description of a case of manufacturing TFTs in adifferent step other than Embodiment 1 with reference to FIGS. 21A to21C. Embodiment 11 is merely different from Embodiment 1 in some stepsand the rest is the same. Therefore the same reference symbols are usedin the identical steps.

First, the second etching treatment and the second doping treatment areconducted in accordance with the manufacture process shown in Embodiment1 to reach the state of FIG. 7C where the second shape conductive layersand the wiring lines are formed.

Next, resist masks 801 and 802 are formed to cover the future n-channelTFT and the future second p-channel TFT, respectively, and a thirdetching treatment is conducted. In the third etching treatment, Cl₂ isused as the etching gas, the gas flow rate thereof is set to 80 SCCM,and an RF (13.56 MHz) power of 350 W is applied to a coiled electrode ata pressure of 1.2 Pa to generate plasma for 40 second etching. Thesubstrate (sample stage) side receives an RF (13.56 MHz) power of 50 Wto apply a substantially negative self-bias voltage. Thus third shapeconductive layers and wiring lines 803 to 807 are formed (FIG. 21B).

After the resist masks 801 and 802 are removed, masks 808 and 809 arenewly formed from a resist to cover the n-channel TFT and the pixel TFT,respectively, A third doping treatment is conducted. Through the thirddoping treatment, the semiconductor layers in the p-channel TFTs aredoped with a p type impurity element while using the third shapeconductive layers and the capacitance wiring lines as masks. Fourthconcentration impurity regions 810 a to 810 c and fifth concentrationimpurity regions 810 d to 810 f are thus formed in a self-aligningmanner (FIG. 21C).

After finishing the steps described above, the active matrix substrateis completed in accordance with the step of forming an inorganicinterlayer insulating film and the following steps thereof disclosed inEmbodiment 1.

Embodiment 12

This embodiment shows results of measuring characteristics of TFTsmanufactured in accordance with manufacture methods disclosed in thisspecification.

First, FIG. 40 shows a graph representing a relation between the draincurrent (Id) and the gate voltage (Vg) (hereinafter referred to as Id-Vgcurve) of a pixel TFT (n-channel TFT) manufactured in accordance withthe manufacture method described in Embodiment 5. The measurement hasbeen made by setting the source voltage (Vs) to 0 V and the drainvoltage (Vd) to 1 V or 14 V. The measured value of the channel length(L) is 6 μm and the measured value of the channel width (W) is 4 μm.

OFF current (Ioff) is 0.5 pA when Vd is 14 V.

Next, FIGS. 41A and 41B respectively show Id-Vg curves of a pixel TFTand a first p-channel TFT of a driver circuit that are obtained throughthe manufacture method described in Embodiment 8.

The measurement has been made by setting the source voltage (Vs) to 0 Vand the drain voltage (Vd) to 1 V or 14 V. The measured value of thechannel length (L) is 6 μm and the measured value of the channel width(W) is 4 μm in the pixel TFT. The measured value of the channel length(L) is 7 urn and the measured value of the channel width (W) is 8 urn inthe first p-channel TFT.

When Vd is 14 V, OFF current (Ioff) of the pixel TFT is 0.3 pA whereasOFF current (Ioff) of the first p-channel TFT is 2 pA. Comparing them toa p-channel TFT that has no offset region, the pixel TFT and the firstp-channel TFT can control sharp rise of Ioff when Vg is high.

An n-channel TFT, a p-channel TFT and a pixel TFT manufactured inaccordance with another embodiment of the invention have also displayedexcellent characteristics. The n-channel TFT has an Ioff of 10 to 30 pA,a field effect mobility of 130 to 180 cm²/Vs, and an S value of 0.19 to0.26 V/dec. The p-channel TFT has an Ioff of 2 to 10 pA, a field effectmobility of 70 to 110 cm²/Vs, and an S value of 0.19 to 0.25 V/dec. Thepixel TFT has an Ioff of 2 to 10 pA, a field effect mobility of 70 to150 cm²/Vs, and an S value of 0.16 to 0.24 V/dec.

Now, results of measurement on reliability will be shown.

The reliability is estimated by checking the ten-year guarantee voltage.The ten-year guarantee voltage is obtained by inferring a stress voltagehaving a lifetime of ten years from a linear relation provided byplotting the reciprocal of a stress voltage into a semi-logarithmicgraph. The lifetime here is defined as a time a TFT takes to change itsmaximum mobility value (μFE_((max))) by 10%. TFTs (driver circuit)manufactured in accordance with the manufacture method of EmbodimentMode 1 have been measured. The ten-year guarantee voltage of the TFTs is20 V or higher as shown in FIG. 42, displaying high reliability.

The thousand-hour life temperature by ON stress is checked next. Thetemperature at which the characteristic changes by 0.1 V in thousandhours (life temperature) is inferred by plotting the time the TFTcharacteristic (Shift #1) takes to change by 0.1 V when Vg is +20 V (−20V in the p-channel TFT) and Vd is 0V against 1000/T (T: absolutetemperature (K)). As shown in FIG. 43, the thousand-hour lifetemperature is 80° C. or higher in both the n-channel TFT and p-channelTFT.

The thousand-hour life temperature by OFF stress is checked next. Thetemperature at which the characteristic changes by 0.1 V in thousandhours (life temperature) is inferred by plotting the time the TFTcharacteristic (Shift #1) takes to change by 0.1 V when Vg is 0V and Vdis +20 V (−20 V in the p-channel TFT) against 1000/T (T: absolutetemperature (K)). As shown in FIG. 44, the thousand-hour lifetemperature is 80° C. or higher in both the n-channel TFT and p-channelTFT.

The characteristic shift of the n-channel TFT and the characteristicshift of the p-channel TFT due to transient stress are checked next. TheON characteristic shift is observed after twenty hours (at roomtemperature) when Vd is +20 V (−20 V in the p-channel TFT) and Vg is 2to 6 V (−6 to −2 V in the p-channel TFT). (The transient stress is astress applied when the drain voltage is set to a certain value and thegate voltage is set to a certain value.)

FIGS. 45A and 45B confirm that the change in maximum ratio of the fieldeffect mobility in twenty hours is limited to 10% or less in both then-channel TFT and p-channel TFT.

These results prove that a manufacture method of the present inventioncan provide highly reliable TFTs having required performances and cangive those excellent TFTs their respective optimal structures withoutincreasing the manufacture steps.

Embodiment 13

The description given in this embodiment with reference to FIG. 22 is ofa process of manufacturing an active matrix liquid crystal displaydevice from an active matrix substrate that is fabricated in accordancewith the process of one of Embodiments 1 and 5 through 11.

An active matrix substrate as shown in FIG. 9C is first prepared usingthe process of one of Embodiments 1 through 8. An alignment film 1181 isformed on the active matrix substrate and subjected to rubbingtreatment. In this embodiment, an organic resin film such as an acrylicresin film is patterned before forming the alignment film 1181 in orderto form in a desired position a columnar spacer 1180 for maintaining adistance between two substrates. Instead of the columnar spacer,spherical spacers may be sprayed onto the entire surface of thesubstrate.

An opposing substrate 1182 is prepared next. Colored layers 1183 and1184 and a leveling film 1185 are formed on the opposing substrate 1182.The red colored layer 1183 partially overlaps the blue colored layer1184 to form a second light shielding portion. Though not shown in FIG.22, the red colored layer partially overlaps a green colored layer toform a first light shielding portion.

Then an opposing electrode 1186 is formed in the pixel portion. Analignment film 1187 is formed on the entire surface of the opposingsubstrate 1182 and subjected to rubbing treatment.

The active matrix substrate on which the pixel portion and the drivercircuit are formed is bonded to the opposing substrate with a sealingmember. The sealing member has a filler mixed therein. The filler,together with the columnar spacer, keeps the distance between the twosubstrates uniform when the substrates are bonded to each other.Thereafter, a liquid crystal material 1188 is injected between thesubstrates and the device is completely sealed by an end-sealingmaterial (not shown). The liquid crystal material 1188 may be a knownliquid crystal material. Thus an active matrix liquid crystal displaydevice shown in FIG. 22 is completed.

The number of manufacture steps can be reduced by forming a first lightshielding portion or a second light shielding portion from coloredlayers to shield gaps between pixels from light as in this embodimentinstead of forming a black mask.

Embodiment 14

FIG. 23 shows a block diagram of a semiconductor device manufactured inaccordance with the present invention. This embodiment describes asemiconductor device having a source side driver circuit 90, a pixelportion 91, and a gate side driver circuit 92. The term driver circuitherein collectively refers to a source side driver circuit and a gateside driver circuit.

The source side driver circuit 90 is provided with a shift register 90a, a buffer 90 b, and a sampling circuit (transfer gate) 90 c. The gateside driver circuit 92 is provided with a shift register 92 a, a levelshifter 92 b, and a buffer 92 c. If necessary a level shifter circuitmay be provided between the sampling circuit and the shift register.

In this embodiment, the pixel portion 91 is composed of a plurality ofpixels and each of the plural pixels has TFT elements.

Though not shown in the drawing, another gate side driver circuit may beprovided in across the pixel portion 91 from the gate side drivercircuit 92.

When the device is digitally driven, the sampling circuit is replaced bya latch (A) 93 b and a latch (B) 93 c as shown in FIG. 24. A source sidedriver circuit 93 is provided with a shift register 93 a, the latch (A)93 b, the latch (B) 93 c, a D/A converter 93 d, and a buffer 93 e. Agate side driver circuit 95 is provided with a shift register 95 a, alevel shifter 95 b, and a buffer 95 c. If necessary a level shiftercircuit may be provided between the latch (B) 93 c and the D/A converter93 d. A reference symbol 94 denotes a pixel portion.

The above structure is obtained by employing the manufacture process ofany of Embodiments 1 through 8. Although this embodiment describes onlythe structure of the pixel portion and the driver circuit, a memorycircuit and a microprocessor circuit can also be formed when followingthe manufacture process of the present invention.

Embodiment 15

This embodiment gives a description with reference to FIGS. 25A to 25Don a process of forming a semiconductor film to serve as an active layerof a TFT. The crystallization means in this embodiment is a techniquedescribed in Embodiment Mode 1 of Japanese Patent Application Laid-openNo. Hei 7-130652.

First, a base insulating film 1402 with a thickness of 200 nm is formedon a substrate (glass substrate, in this embodiment) 1401 from a siliconoxynitride film. An amorphous semiconductor film (amorphous siliconfilm, in this embodiment) 1403 with a thickness of 200 nm is formedthereon. The base insulating film and the amorphous semiconductor filmmay be formed successively without exposing them to the air.

Next, an aqueous solution containing 10 ppm of catalytic element byweight (in this embodiment, the catalytic element is nickel and theaqueous solution is nickel acetate aqueous solution) is applied by spincoating to form a catalytic element containing layer 1404 over theentire surface of the amorphous semiconductor film 1403. Examples of thecatalytic element that can be used here other than nickel (Ni) includeiron (Fe), palladium (Pd), tin (Sn), lead (Pb), cobalt (Co), platinum(Pt), copper (Cu), and gold (Au) (FIG. 25A).

Although spin coating is used in doping of nickel in this embodiment, acatalytic element may be deposited by evaporation or sputtering to forma thin film (nickel film, in the case of this embodiment) on theamorphous semiconductor film.

Prior to the crystallization step, heat treatment is conducted at 400 to500° C. for about an hour to release hydrogen from the film. Then thefilm is subjected to heat treatment at 500 to 650° C. (preferably 550 to570° C.) for four to twelve hours (preferably four to six hours). Inthis embodiment, the film is heated at 550° C. for four hours to form acrystalline semiconductor film (crystalline silicon film, in thisembodiment) 1405 (FIG. 25B).

A laser light irradiation step may be inserted here to improve thecrystallinity of the crystalline semiconductor film 1405.

The next step is gettering for removing nickel used in thecrystallization step from the crystalline silicon film. First, a maskinsulating film 1406 with a thickness of 150 nm is formed on the surfaceof the crystalline semiconductor film 1405 and is patterned to form anopening 1407. Then the exposed portion of the crystalline semiconductorfilm is doped with an element belonging to Group 15 (phosphorus, in thisembodiment). Through this step, a gettering region 1408 containingphosphorus in a concentration of 1×10¹⁹ to 1×10²⁰ atoms/cm³ is formed(FIG. 25C).

A heat treatment step is carried out next in a nitrogen atmosphere at450 to 650° C. (preferably 500 to 550° C.) for four to twenty-four hours(preferably six to twelve hours). Through the heat treatment step,nickel in the crystalline semiconductor film moves in the directionindicated by the arrow and is trapped in the gettering region 1408 bythe gettering action of phosphorus. Since nickel is removed from thecrystalline semiconductor film, the concentration of nickel contained inthe crystalline semiconductor film 1409 is reduced to 1×10¹⁷ atoms/cm³or lower, preferably 1×10¹⁶ atoms/cm³ (FIG. 25D).

The crystalline semiconductor film 1409 formed as above has a very highcrystallinity owing to the use of a catalytic element for promotingcrystallization (nickel, in this embodiment).

An alternative method of gettering the catalytic element is to utilizephosphorus (P) as the n type impurity element for doping the sourceregion or the drain region in the step of activating the impurityelement used to dope the semiconductor film after the inorganicinterlayer insulating film is formed in the manufacture process ofEmbodiment 1.

The structure of this embodiment can be combined freely with thestructure shown in Embodiment Mode 1 and Embodiments 1 through 8.

Embodiment 16

This embodiment gives a description with reference to FIGS. 26A to 26Don a process of forming a semiconductor film to serve as an active layerof a TFT. Specifically, a technique described in Japanese PatentApplication Laid-open No. Hei 10-247735 (corresponding to U.S. Pat. No.6,165,824) is used.

First, a base insulating film 1502 with a thickness of 200 nm is formedon a substrate (glass substrate, in this embodiment) 1501 from a siliconoxynitride film. An amorphous semiconductor film (amorphous siliconfilm, in this embodiment) 1503 with a thickness of 200 nm is formedthereon. The base insulating film and the amorphous semiconductor filmmay be formed successively without exposing them to the air.

A mask insulating film 1504 is then formed from a silicon oxide film toa thickness of 200 nm. An opening 1505 is formed in the film.

Next, an aqueous solution containing 100 ppm of catalytic element byweight (in this embodiment, the catalytic element is nickel and theaqueous solution is nickel acetate aqueous solution) is applied by spincoating to form a catalytic element containing layer 1506. At thispoint, the catalytic element containing layer 1506 selectively contactsthe amorphous semiconductor film 1503 in the region where the opening1505 has been formed. Examples of the catalytic element that can be usedhere other than nickel (Ni) include iron (Fe), palladium (Pd), tin (Sn),lead (Pb), cobalt (Co), platinum (Pt), copper (Cu), and gold (Au) (FIG.26A).

Although spin coating is used in doping of nickel in this embodiment, acatalytic element may be deposited by evaporation or sputtering to forma thin film (nickel film, in the case of this embodiment) on theamorphous semiconductor film.

Prior to a crystallization step, heat treatment is conducted at 400 to500° C. for about an hour to release hydrogen from the film. Then thefilm is subjected to heat treatment at 500 to 650° C. (preferably 550 to600° C.) for six to sixteen hours (preferably eight to fourteen hours).In this embodiment, the film is heated at 570° C. for fourteen hours. Asa result, crystallization starts from the opening 1505 and progresses ina direction substantially parallel to the substrate (the directionindicated by the arrow) to form a crystalline semiconductor film(crystalline silicon film, in this embodiment) 1507 (FIG. 26B).Macroscopically, the crystal growth direction of the crystallinesemiconductor film 1507 is uniform.

The next step is gettering for removing nickel used in thecrystallization step from the crystalline silicon film. In thisembodiment, the mask insulating film 1504 previously formed is used as amask without changing anything about the insulating film and thecrystalline semiconductor film is doped with an element belonging toGroup 15 (phosphorus, in this embodiment). A gettering region 1508 isformed as a result in the exposed part of the crystalline semiconductorfilm at the opening 1505. The gettering region 1508 contains phosphorusin a concentration of 1×10¹⁹ to 1×10²⁰ atoms/cm³ is formed (FIG. 26C).

A heat treatment step is carried out next in a nitrogen atmosphere at450 to 650° C. (preferably 500 to 550° C.) for four to twenty-four hours(preferably six to twelve hours). Through the heat treatment step,nickel in the crystalline semiconductor film moves in the directionindicated by the arrow and is trapped in the gettering region 1;0S bythe gettering action of phosphorus. Since nickel is removed from thecrystalline semiconductor film, the concentration of nickel contained inthe crystalline semiconductor film 1509 is reduced to 1×10¹⁷ atoms/cm orlower, preferable 1×10¹⁶ atoms/cm³ (FIG. 26D).

The crystalline semiconductor film 1509 formed as above has a very highcrystallinity by being crystallized while selectively doped with acatalytic element for promoting crystallization (nickel, in thisembodiment). Specifically, the film has a crystal structure in whichrod-like or columnar crystals are arranged in a specific orientation.

An alternative method of gettering the catalytic element is to utilizephosphorus (P) as the n type impurity element for doping the sourceregion or the drain region in the step of activating the impurityelement used to dope the semiconductor film after the inorganicinterlayer insulating film is formed in the manufacture process ofEmbodiment 1.

The structure of this embodiment can be combined freely with thestructure shown in Embodiment Mode 1 and Embodiments 1 through 8.

Embodiment 17

Described below using FIG. 3A to FIG. 6 (each corresponding to FIG. 27Ato FIG. 30) is a method of manufacturing a semiconductor device in whicha TFT for a pixel portion and a TFT for a driver circuit provided in theperiphery of the pixel portion are formed on the same substrate. Thesemiconductor device has a pixel electrode that is uneven because of anuneven region formed in the pixel portion by the same manufactureprocess as the TFTs.

A substrate 2100 in this embodiment is made of glass such as bariumborosilicate glass or alumino borosilicate glass, typical example ofwhich is Coming # 7059 or # 1737 glass (a product of CorningIncorporated). The substrate 2100 may be a quartz substrate, a siliconsubstrate, a metal substrate, or a stainless steel substrate if aninsulating film is formed on the surface. A plastic substrate may alsobe used if it has a heat resistance against the process temperature ofthis embodiment.

On the surface of the substrate 2100, a base insulating film 2101 isformed from an insulating film such as a silicon oxide film, a siliconnitride film, and a silicon oxynitride film. In this embodiment, thefirst layer of the base insulating film 2101 is a silicon oxynitridefilm (composition ratio: Si=32%. O=27%. N=24%, H=17%) 2101 a formed to athickness of 10 to 200 nm (preferably 50 to 100 nm) by plasma CVD usingas reaction gas SiH₄, NH₃, and N₂O. The second layer of the baseinsulating film is a silicon oxynitride film (composition ratio: Si=32%,O=59%, N=7%, H=2%) 2101 b formed to a thickness of 10 to 200 nm(preferably 100 to 150 nm) by plasma CVD using as reaction gas SiH₄ andN₂O. The second layer is layered on the first layer.

An amorphous semiconductor film is next formed on the base insulatingfilm by a known method (such as sputtering, LPCVD, or plasma CVD). Theamorphous semiconductor film is then crystallized by a knowncrystallization treatment (laser crystallization, thermalcrystallization, or thermal crystallization using Ni or other catalyticelement) to form a crystalline semiconductor film. The obtainedcrystalline semiconductor film is patterned into a desired shape to formisland-like semiconductor layers 2102 to 2105 and an island-likesemiconductor layer 2301 for forming projections in the pixel portion(See FIG. 3A). Hereinafter, the projections in this embodiment areformed in accordance with the process of manufacturing a pixel TFT.

No limitation is put on the material of the crystalline semiconductorfilm, but the film is preferably formed of silicon or a silicongermanium (Si_(x)Ge_(1−x); 0<x<1, typically x=0.001 to 0.05) alloy.

In this embodiment, an amorphous silicon film with a thickness of 55 nmis formed by plasma CVD and then irradiated with laser to form acrystalline silicon film. When the semiconductor film is crystallized bylaser treatment, the film is desirably subjected to heat treatment at400 to 500° C. for about an hour in order to reduce the hydrogen contentin the film to 5 atom % or less prior to the crystallization step.

Another employable crystallization method consists of applying asolution containing Ni to the amorphous silicon film, subjecting thefilm to thermal crystallization treatment (at 550° C. for four hours),and performing laser annealing treatment on the obtained crystallinesilicon film to improve crystallinity of the film. Examples of the laserusable in the laser annealing treatment include pulse oscillation typeor continuous wave KrF excimer laser, XeCl excimer laser. YAG laser, andYVO₄ laser. When one of these lasers is used, laser beams emitted from alaser emitter are collected by an optical system into a linear beam toirradiate the semiconductor film. Conditions for crystallization can beset by an operator suitably.

Other crystallization methods than the thermal crystallization involvingdoping of a catalytic element may be employed; the semiconductor filmmay be crystallized by heat without using a catalytic element, or by RTA(rapid thermal annealing) in which the film is crystallized around 500to 700° C. After the semiconductor film is crystallized by RTA, the filmmay be subjected to laser annealing treatment to improve itscrystallinity.

The semiconductor layers may be doped with a minute amount of impurityelement (boron or phosphorus: in this embodiment, boron is used) inorder to control threshold of the TFTs.

Next, a gate insulating film 2106 is formed so as to cover thesemiconductor layers 2102 to 2105 and the island-like semiconductorlayer 2301 for forming the projections. The gate insulating film 2106 isan insulating film containing silicon which is formed by plasma CVD orsputtering to a thickness of 40 to 150 nm. In this embodiment, a siliconoxynitride film (composition ratio: Si=32%, O=59%, N=7%, H=2%) is formedto a thickness of 110 nm by plasma CVD. Needless to say, the gateinsulating film is not limited to a silicon oxynitride film but may be asingle layer or a laminate of other insulating films containing silicon.

If a silicon oxide film is used, the film is formed by plasma CVDthrough electric discharge while using a mixture of TEOS (tetraethylorthosilicate) and O₂, and setting the reaction pressure to 40 Pa, thesubstrate temperature to 300 to 400° C., and the power density to 0.5 to0.8 W/cm² at a high frequency (13.56 MHz). The silicon oxide film formedin this way can provide excellent characteristics as the gate insulatingfilm when subjected to thermal annealing at 400 to 500° C.

Formed next on the State insulating film 2106 are a first conductivefilm 2107 with a thickness of 20 to 100 nm and a second conductive film2108 with a thickness of 100 to 400 nm. In this embodiment, the film2107 is a TaN film having a thickness of 30 nm and the film 2108 is a Wfilm having a thickness of 370 nm. The TaN film is formed by sputteringin an atmosphere containing nitrogen using a Ta target. The W film isformed by sputtering using a W target. Alternatively, the W film may beformed by thermal CVD using tungsten hexafluoride (WF₆).

In either case, the W film has to be less resistive in order to use thefilm for a gate electrode. The resistivity of the W film is desirably 20μΩcm or lower. The W film can have low resistivity when the grain sizeis large. However, if the W film contains many impurity elements such asoxygen, crystallization is hindered and the resistivity is raised.Therefore, the W film in this embodiment is formed by sputtering using ahighly pure W target (purity: 99.9999%) and taking great care not toallow impurities from the air to mix in the film in the middle offormation. A resistivity of 9 to 20 μΩcm is thus attained.

Although the first conductive film 2107 is a TaN film whereas the secondconductive film 2108 is a W film in this embodiment, they are notparticularly limited. Each of the conductive films can be formed of anelement selected from the group consisting of Ta, W, Ti, Mo, Al, and Cu,or may be formed of an alloys material or compound material containingany of the above elements as its main ingredient. Alternatively, asemiconductor film, typically a polycrystalline silicon film, doped withan impurity element such as phosphorus may be used. The first conductivefilm and the second conductive film can take various combinations, e.g.,a combination of Ta film for the first conductive film 2107 and W filmfor the second conductive film 2108, a combination of TaN film for thefirst conductive film 2107 and Al film for the second conductive film2108, and a combination of TaN film for the first conductive film 2107and Cu film for the second conductive film 2108 (FIG. 27A).

Next, masks 2109 to 2113 and a mask 2302 for forming the projections areformed from a resist by photolithography to conduct a first etchingtreatment for forming electrodes and wiring lines. This embodimentemploys ICP (inductively coupled plasma) etching in which CF₄, Cl₂, andO₂ are used as the etching gas, the gas flow rate ratio of them is setto 25,25/10 SCCM, and an RF (13.56 MHz) power of 500 W is applied to acoiled electrode at a pressure of 1.0 Pa to generate plasma for theetching. The substrate side (sample stage) receives an RF (13.56 MHz)power of 150 W to apply a substantially negative self-bias voltage.

Thereafter, etching is made under the second etching conditions withoutremoving the resist masks 2109 to 2113. According to the second etchingconditions. CF₄ and Cl₂ are used as the etching gas, the gas flow rateratio of them is set to 30/30 SCCM, and an RF (13.56 MHz) power of 500 Wis applied to a coiled electrode at a pressure of 1 Pa to generateplasma for 30 second etching. The substrate side (sample stage) alsoreceives an RF (13.56 MHz) power of 20 W to apply a substantiallynegative self-bias voltage. The TaN film and the W film are etched tothe same extent under the second etching conditions using a mixture ofCF₄ and Cl₂. Up to this point, first shape conductive layers 2114 to2118 and a conductive film 2303 for forming the projections are formed.

A first doping treatment is conducted next without removing the resistmasks 2109 to 2113. In the first doping treatment, the semiconductorlayers are doped with an impurity element imparting n-type conductivity(hereinafter referred to as n type impurity element) in a self-aligningmanner while using the first shape conductive layers as masks. Thedoping treatment is achieved by ion doping or ion implantation. The ntype impurity element to be used is an element belonging to Group 15 inthe periodic table, typically, phosphorus (P) or arsenic (As). Throughthe doping, a first concentration impurity region 2120 having aconcentration of 1×10²⁰ to 1×10²¹ atoms/cm³ is formed (FIG. 3B, FIG.27B).

Still keeping the resist masks 2109 to 2113 in place, a second etchingtreatment is conducted. CF₄, Cl₂, and O₂ are used as the etching gas,the gas flow rate ratio of them is set to 20/20/20 SCCM, and an RF(13.56 MHz) power of 500 W is applied to a coiled electrode at apressure of 1 Pa to generate plasma for the etching. The substrate side(sample stage) receives an RF (13.56 MHZ) power of 20 W to apply aself-bias voltage lower than in the first etching treatment. The W filmis etched under these second etching conditions. As a result, secondshape conductive layers 2121 to 2125 and a conductive film 2304 forforming the projections are formed (FIG. 3C).

Then a second doping treatment is conducted. Using as a mask the secondshape first conductive film formed in the first doping treatment, secondconcentration impurity regions 2126 b to 2129 b are formed on the insideof the n type impurity region 2126 a to 2129 a (on the channel formationregion side). The second concentration impurity regions each contain animpurity element in a concentration of 1×10¹⁵ to 1×10¹⁹ atoms/cm².

Next, the resist masks 2109 to 2113 are removed and a mask 2130 is newlyformed from a resist to conduct a third etching treatment. Cl₂ is usedas the etching gas, the gas flow rate thereof is set to 80 SCCM, and anRF (13.56 MHz) power of 350 W is applied to a coiled electrode at apressure of 1.2 Pa to generate plasma for 40 second etching. Thesubstrate side (sample stage) receives an RF (13.56 MHz) power of 50 Wto apply a substantially negative self-bias voltage. Thus the secondshape Crate electrodes in the future p-channel TFT of the driver circuitand in the future pixel TFT are etched to form third shape gateelectrodes 2131 and 2132 of the future p-channel TFT and the futurepixel TFT, respectively, and to form a conductive film 2305 for formingthe projections (FIG. 4B. FIG. 28B). In this specification, a ‘futurepixel TFT’ refers to a pixel TFT in the middle of fabrication.Similarly, a ‘future n-channel TFT’ (‘future p-channel TFT’) refers toan unfinished TFT that is to function as an n-channel TFT (p-channelTFT) after its completion.

A resist mask 2133 is newly formed to cover the future pixel TFT and theuneven region. The future n-channel TFT of the driver circuit is coveredwith the mask 2130. Then a third doping treatment is conducted to dopethe semiconductor layers in the p-channel TFT and in the storagecapacitor with an impurity element imparting p-type conductivity(hereinafter referred to as p type impurity element). In thisembodiment, the semiconductor layers are doped with a p type impurityelement in a self-aligning manner while using the third shape conductivelayers as masks to form fourth concentration impurity regions. Thisembodiment employs ion doping using diborane (B₂H₆) to form fourthconcentration impurity regions 2134 to 2137.

The fourth concentration impurity regions are doped with an n typeimpurity element (phosphorus (P), in this embodiment) in differentconcentrations. However, all of them do not have a problem to functionas source regions and drain regions of the p-channel TFTs because dopingof impurity elements is performed making sure that those impurityregions contain the p type impurity element in a concentration higherthan the concentration of the n type impurity element.

Through the above steps, the respective semiconductor layers are dopedwith the impurity elements for imparting the respective conductivitytypes and all of the impurity regions are formed in a self-aligningmanner while using the gate electrodes as masks.

The plural projections formed in the pixel portions are obtained throughsteps identical with the steps of forming the pixel TFT.

The resist masks 2130, 2133, and 2134 are removed and a first interlayerinsulating film 2138 is formed to cover the entire surface. In order tomake the insulating film susceptive to an uneven region 1207 formed inthe pixel portion, the first interlayer insulating film 2138 is formedfrom an insulating film containing silicon by plasma CVD or sputteringto a thickness of 200 to 400 nm. In this embodiment, a siliconoxynitride film with a thickness of 400 nm is formed by plasma CVD. Thematerial of the insulating film is not limited to a silicon oxynitridefilm and a single layer or a laminate of other insulating filmscontaining silicon may be used.

The next step is heat treatment for activating the impurity elementsused to dope the semiconductor layers. This heat treatment step foractivation is achieved by heat treatment that uses a furnace (furnaceannealing). Conditions of the heat treatment includes preparing nitrogenatmosphere whose oxygen concentration is 1 ppm or less, preferably, 0.1ppm or less, and setting the temperature to 300 to 500° C., typically400 to 450° C. In this embodiment, activation is made by heat treatmentat 450° C. for four hours. Other than furnace annealing, laserannealing, RTA, or thermal annealing may be adopted.

If a catalytic element is used in crystallization, the concentration ofNi used as a catalyst has to be lowered in the channel formation region.Then gettering and the heat treatment activation are simultaneouslyconducted, so that nickel is moved to an n type impurity region thatcontains a high concentration of phosphorus (P). In this case, thetemperature of the heat treatment is set to 300 to 700° C., typically500 to 550° C. Thus the nickel concentration can be lowered in thesemiconductor layer most part of which is to serve as the channelformation region. If a TFT has a channel formation region formed asabove, the OFF current value thereof is low and crystallinity is high toprovide high field effect mobility, whereby the TFT can have excellentcharacteristics.

The heat treatment for activation in this embodiment is conducted afterthe first interlayer insulating film 2138 is formed. However, the firstinterlayer insulating film 2138 may be formed after the heat treatment.If the material used for the conductive films is weak against heat, itis preferred to form the interlayer insulating film for protecting theconductive films before the heat treatment step as in this embodiment.

The semiconductor layers are subjected to another heat treatment in anatmosphere containing 3 to 100% of hydrogen at 300 to 550° C. for one totwelve hours for hydrogenation. In this embodiment, heat treatment isconducted in an atmosphere containing about 3% of hydrogen at 410° C.for an hour. This step is to terminate dangling bonds in thesemiconductor layers by hydrogen contained in the interlayer insulatingfilm. Other hydrogenation measures include plasma hydrogenation(utilizing hydrogen excited by plasma).

In the case where the activation process is carried out by laserannealing, it is desirable to add laser irradiation by excimer laser,YAG laser, or the like after the hydrogenation described above.

An alternative is to form a silicon oxynitride film with a thickness of50 to 100 nm as the first interlayer insulating film 2138, conduct heattreatment at 300 to 700° C. (typically 550° C.) for about four hours foractivation of the impurity elements used to dope the semiconductor film,form a silicon nitride film to a thickness of 100 to 300 nm, and conductanother heat treatment at 300 to 550° C. for one to twelve hours in anitrogen atmosphere containing hydrogen.

Next, a second interlayer insulating film 2139 is formed on the firstinterlayer insulating film 2138. In this embodiment, an acrylic resinfilm is formed to a thickness of 0.8 to 1.2 μm. Influenced by the unevenregion formed in the pixel portion, the second interlayer insulatingfilm 2139 has uneven surface. The interlayer insulating film may beformed without removing the resist mask used to form the protrusions inorder to make the influence of the protrusions clearer.

Then contact holes reaching the source wiring lines and thesemiconductor layers (impurity regions) of the TFTs are formed throughthe first interlayer insulatine film 2138 and the second interlayerinsulating film 2139.

Wiring lines 2140 to 2145 for electrically connecting the TFTs areformed next. The wiring lines 2140 to 2145 are formed by patterning alaminate of a Ti film with a thickness of 50 to 250 nm and an alloy film(an alloy film of Al and Ti) with a thickness of 300 to 500 nm.

A pixel electrode 2144 is formed in the pixel portion. The pixelelectrode 2144 is desirably formed of a material having excellentreflectivity, such as a film mainly containing Al or Ag, and a laminateof a Al containing film and a Ag containing film. Influenced by theuneven region 1207 formed in a pixel portion 1206, the pixel electrodeis uneven.

In this embodiment, an end of the pixel electrode 2144 overlaps a sourceline with the first interlayer insulating film 2138 and the secondinterlayer insulating film 2139 interposed therebetween. Therefore gapsbetween pixel electrodes can be shielded from light without using ablack mask.

In this way, a driver circuit 1205 that has an n-channel TFT 1201(channel formation region 2146) and a p-channel TFT 1202 (channelformation region 2147) is formed on the same substrate on which thepixel portion 1206 having a pixel TFT 1203 (channel formation region2148), a storage capacitor 1204, and the uneven region 1207 is formed(FIG. 29B). A substrate as such is called an active matrix substrate inthis specification.

FIG. 30 shows the top view of the active matrix substrate manufacturedin accordance with this embodiment. In the case shown in thisembodiment, a source line 2125 and a gate electrode are formed from thesame conductive film in the same layer (the gate insulating film 2119).The pixel portion in this embodiment is provided with the uneven region1207.

The manufacture process shown in this embodiment requires only six photomasks to fabricate an active matrix substrate (namely, a semiconductorlayer pattern mask, a mask for forming a gate electrode, a mask foretching an unnecessary L_(ov) region, a mask for forming for forming asource region and a drain region of a p-channel, a mask for formingcontact holes, and a mask for forming a wiring line and a pixelelectrode). Therefore a reflective active matrix substrate in which anuneven region having a plurality of protrusions is formed in a pixelportion to form an uneven pixel electrode can be manufactured withoutcomplicating the manufacture process. This embodiment is thus capable ofcontributing to cutting manufacture cost and improving the yield.

Embodiment 18

A reflective liquid crystal display device will be described in which anelectro-optical device manufactured employing the present invention iscombined with a light source, a reflector, and a light guide plate.

An LED or a cold-cathode tube is used for the light source. The lightsource is arranged along a side face of the light guide plate. Thereflector is placed behind the light source. In this specification, thetop face of the light guide plate refers to the face facing a user andthe bottom face of the light guide plate refers to the face opposite tothe top face.

As shown in FIG. 46, light emitted from the light source efficientlyenters the interior from the side face of the light guide plate owing tothe reflector. The incident light is reflected at a part of the surfacewhich is processed to form a prism and enters and travels through thesemiconductor device. The light is then reflected at a reflective filmprovided on the bottom face of the semiconductor device, and goes backthrough the electro-optical device and the light guide plate to reacheves of the user.

The material of the light guide plate may be quarts, inorganic glass(refractive index: 1.42 to 1.7, transmissivity: 80 to 91%) such asborosilicate glass, or a plastic material (resin material). The usableplastic material is a mixture of resins such as a methacrylic resin,typically polymethylmethacrylate known as acryl (refractive index: 1.49,transmissivity: 92 to 93%), polycarbonate (refractive index: 1.59,transmissivity: 88 to 90%), polyarylate (refractive index: 1.61,transmissivity: 85%), poly-4-methylpentene-1 (refractive index: 1.46,transmissivity: 90%), an AS resin [acrylonitrile-styrene polymer](refractive index: 1.57, transmissivity: 90%), and an MS resin[methylmethacrylate-styrene copolymer] (refractive index: 1.56,transmissivity: 90%).

A semiconductor device manufactured in accordance with any one ofEmbodiments 1 through 11 can be applied to this embodiment.

Embodiment 19

In the top view of FIG. 47A, an opposing substrate 2151 provided with acolor filter and other components is bonded to an active matrixsubstrate through a sealing member. The active matrix substrate isprovided with a pixel portion, a driver circuit, an external inputterminal 210 for bonding an FPC (flexible printed circuit), and aconnection wiring line 2211 for connecting the external input terminalto input portions of circuits.

The FPC is composed of a base film 2213 and a wirings line 2214, and isbonded to the external input terminal by anisotropic conductive resin2215. The mechanical strength of the bonding is enhanced by areinforcing plate.

FIG. 47B shows a sectional view of the external input terminal 2210taken along the line e–e′ in FIG. 47A. Denoted by 2217 is a wiring lineformed of a conductive film to form a pixel electrode 2144. The outerdiameter of a conductive particle 2216 is smaller than the pitch of thewiring line 2217. Therefore, when dispersed throughout the adhesive 2215in an appropriate amount, the conductive particle can establish anelectric connection with the corresponding wiring line on the FPC sidewithout causing short-circuit with adjacent wiring lines.

The liquid crystal display panel manufactured as above can be used for adisplay unit of various electric appliances.

Embodiment 20

This embodiment describes a case in which pixel TFTs for a pixel portionof a semiconductor device and TFTs for driver circuit of thesemiconductor device all have the same conductivity type (all of themare p-channel TFTs, or all of them are n-channel TFTs). The descriptionis given with reference to FIGS. 31A and 31B.

A general driver circuit is designed based on a CMOS circuit in which ann-channel TFT and a p-channel TFT are combined complementarily. On theother hand, the driver circuit of this embodiment is composed solely ofTFTs having the same conductivity type (p-channel TFTs). Accordingly,the mask used in doping an impurity element for controlling theconductivity type is unnecessary, and one less masks can be accomplishedin the manufacturing process of the TFTs. As a result, cutting themanufacture process and manufacture cost is made possible.

In a PMOS circuit, there are an EEMOS circuit composed of enhancementtype TFTs and an EDMOS circuit composed of a combination of anenhancement type TFT and a depletion type TFT.

An example of the EEMOS circuit is shown in FIG. 31A whereas an exampleof the EDMOS circuit is shown in FIG. 31B. In FIG. 31A, denoted by 1801and 1802 are both enhancement type p-channel TFTs (hereinafter referredto as E type PTFT). In FIG. 31B, 1803 denotes an E type TFT while 1804denotes a depletion type p-channel TFT (hereinafter referred to as Dtype PTFT).

In FIGS. 31A and 31B, V_(DH) denotes a power supply line to which apositive voltage is applied (positive power supply line) and V_(DL)denotes a power supply line to which a negative voltage is applied(negative power supply line). The negative power supply line may be apower supply line of a ground electric potential (ground power supplyline).

As described above, the steps of forming an n-channel TFT are eliminatedwhen all the TFTs are p-channel TFTs, thereby simplifying themanufacture process of an active matrix liquid crystal display device.Accompanying the simplification, the yield in the manufacture process isimproved and manufacture cost of the active matrix liquid crystaldisplay device can be reduced.

The characteristic required for a TFT varies depending on which circuitthe TFT constitutes. By combining Embodiments 1 through 8. TFTs havingdifferent structures can be formed for different circuits withoutincreasing the number of manufacture steps.

Embodiment 21

A semiconductor device manufactured in accordance with Embodiments 1through 8 employs the GOLD structure that is known to be effective inpreventing degradation of the ON current value due to hot carriers inorder to secure reliability of a TFT of a driver circuit.

The present inventors have conducted tests on reliability in which theoptimum value is obtained for the length of a region where a gateelectrode and a low concentration impurity region overlap in the channellength direction in the GOLD structure (the length is hereinafter calledthe length of the L_(ov) region) by setting three kinds of L_(ov) lengthconditions.

The characteristic shift of an n-channel TFT due to transient stress ischecked. The ON characteristic shift is observed after twenty hours (atroom temperature) when Vd is +20 V and Vg is 2 to 6 V. The transientstress is a stress applied when the drain voltage is set to a certainvalue and the gate voltage is set to a certain value. The presentinventors use the transient stress to estimate the reliability of a TFT.

FIG. 32 shows results of measuring the transient stress of sampleshaving different L_(ov) lengths. The results in FIG. 32 confirm that thechange in maximum value of the field effect mobility in twenty hours islimited to 10% or less when the L_(ov) length is 1 μm or longer.

Subsequently, the time the current degradation rate takes to reach 10%is plotted against the reciprocal of the drain voltage. The ten-yearguarantee voltage is obtained by inferring a stress voltage having alifetime of ten years from a linear relation provided by plotting thereciprocal of a stress voltage into a semi-logarithmic graph. Thelifetime here is defined as a time a TFT takes to change its maximummobility value (μFE_((max))) by 10%. The present inventors use theten-year guarantee voltage to estimate the reliability of a TFT.

FIG. 33 shows results of obtaining the ten-year guarantee voltage forvarying L_(ov) lengths. The results in FIG. 33 show that a highlyreliable semiconductor device can be obtained when the length of theL_(ov) region is 1 μm or longer, preferably, 1.5 μm or longer.

Embodiment 22

The CMOS circuit and the pixel portion formed by implementing thepresent invention can be used in an active matrix liquid crystal displaydevice. Namely, the present invention can be implemented for allelectronic equipment that incorporates the semiconductor device (liquidcrystal display device) in its display portion.

The following can be given as such electronic equipment: a video camera,a digital camera, a projector (rear type or front type), a head mounteddisplay (goggle type display), a personal computer, and a portableinformation terminal (such as a mobile computer, a portable telephone,or an electronic book). Some examples of these are shown in FIGS. 34A to36C.

FIG. 34A shows a personal computer, which contains components such as amain body 5001, an image input portion 5002, a display portion 5003, anda keyboard 5004. The present invention can be applied to the image inputportion 5002, the display portion 5003, and other signal controlcircuits.

FIG. 34B shows a video camera, which contains components such as a mainbody 5101, a display portion 5102, an audio input portion 5103,operation switches 5104, a battery 5105, and an image receiving portion5106. The present invention can be applied to the display portion 5102,and other signal control circuits.

FIG. 34C shows a mobile computer, which contains components such as amain body 5201, a camera portion 5202, an image receiving portion 5203,operation switches 5204, and a display portion 505. The presentinvention can be applied to the display portion 5205 and other signalcontrol circuits.

FIG. 34D shows a goggle type display, which contains components such asa main body 5301, a display portion 5302, and arm portions 5303. Thepresent invention can be applied to the display portion 5302 and othersignal control circuits.

FIG. 34E shows a player which uses a recording medium with a programrecorded therein (hereinafter referred to as a recording medium), whichcontains components such as a main body 5401, a display portion 5402, aspeaker portion 5403, a recording medium 5404, and operation switches5405. Note that a DVD (digital versatile disk) or CD (compact disk) isused as the recording medium for this player, and that appreciation ofmusic or a movie or performing games or the Internet can be done. Thepresent invention can be applied to the display portion 5402 and othersignal control circuits.

FIG. 34F shows a digital camera, which contains components such as amain body 5501, a display portion 5502, an eve piece portion 5503,operation switches 5504, and an image receiving portion (not shown inthe figure). The present invention can be applied to the display portion5502 and other signal control circuits.

FIG. 35A shows a front type projector, which contains components such asa projecting apparatus 5601 and a screen 5602. The present invention canbe applied to a liquid crystal display device 5808 which structures aportion of the projecting apparatus 5601, and to other signal controlcircuits.

FIG. 35B shows a rear type projector, which contains components such asa main body 5701, a projecting apparatus 5702, a mirror 5703, and ascreen 5704. The present invention can be applied to the liquid crystaldisplay device 5808 which structures a portion of the projectingapparatus 5702, and to other signal control circuits.

Note that an example of the structure of the projecting apparatuses 5601and 5702 of FIG. 35A and FIG. 35B is shown in FIG. 35C. The projectingapparatuses 5601 and 5702 are each composed of a light source opticalsystem 5801, mirrors 5802 and 5804 to 5806, a dichroic mirror 5803, aprism 5807, the liquid crystal display device 5808, a phase differenceplate 5809, and a projecting optical system 5810. The projecting opticalsystem 5810 is composed of an optical system including a projectionlens. A three-plate type example is shown in Embodiment 10, but thereare no particular limitations, and a single-plate type may also be used,for example. Further. optical systems such as an optical lens, a filmhaving a light polarizing function, a film for regulating the phasedifference, and an IR film may be suitably placed in the optical pathshown by the arrow in FIG. 35C by the operator.

Furthermore. FIG. 35D is a diagram showing one example of the lightsource optical system 5801 in FIG. 35C. In Embodiment 22, the lightsource optical system 5801 is composed of a reflector 5811, a lightsource 5812, lens arrays 5813 and 5814, a polarizing conversion element5815, and a condenser lens 5816. Note that the light source opticalsystem shown in FIG. 35D is one example, and the light source opticalsystem is not limited to the structure shown in the figure. For example,optical systems such as an optical lens, a film having a lightpolarizing function, a film for regulating the phase difference, and anIR film may be suitably added to the light source optical system by theoperator.

Note that a case using a transmitting type electro-optical device in theprojectors shown in FIG. 35A is shown here, and examples of applying areflecting type electro-optical device and EL display device are notshown in the figures.

FIG. 36A shows a portable telephone, and reference numerals 3001 and3002 denote a display panel and an operation panel, respectively. Thedisplay panel 3001 and the operation panel 3002 are connected through aconnecting portion 3003. In the connecting portion 3003, an angle eformed by the surface on which a display portion 3004 of the displaypanel 3001 is provided and the surface on which operation keys 3006 ofthe operation panel 3002 are provided can be arbitrarily changed.Further, the portable telephone includes an audio output portion 3005,the operation keys 3006, a power source switch 3007, and an audio inputportion 3008. The present invention can be applied to the displayportion 3004.

FIG. 36B shows a portable book (electronic book), which containscomponents such as a main body 3101, display portions 3102 and 3103, arecording medium 3104, operation switches 3105, and an antenna 3106. Thepresent invention can be applied to the display portions 3102 and 3103,and to other signal control circuits.

FIG. 36C shows a display, which contains components such as a main body3201, a support stand 3202, and a display portion 3203. The presentinvention can be applied to the display portion 3103. The display of thepresent invention is advantageous for cases of large size screens inparticular, and is advantageous for displays having a diagonal equal toor greater than 10 inches (in particular, equal to or greater than 30inches).

The applicable range of the present invention is thus extremely wide,and the present invention can be applied to electronic equipment of allfields. Furthermore, the electronic equipment in this embodiment can berealized by using a semiconductor device formed in accordance with anycombination of Embodiments 1 to 14.

By employing the present invention. TFTs whose required characteristicsare different from one another can be formed on the same substratewithout increasing the number of manufacture steps. Since themanufacture steps do not increase, manufacture cost is lowered and theyield is not reduced. The present invention also can provide a highlyreliable semiconductor device.

Moreover, the semiconductor device can have excellent visibility bygiving its pixel electrode an uneven surface.

1. A portable information terminal having a display device, the displaydevice comprising: a pixel portion formed over a substrate, the pixelportion comprising a first n-channel thin film transistor; and a drivercircuit portion formed over the substrate, the driver circuit portioncomprising a second n-channel thin film transistor and a p-channel thinfilm transistor, wherein the second n-channel thin film transistorcomprises a first semiconductor layer and a first gate electrode with afirst gate insulating film interposed therebetween, wherein thep-channel thin film transistor comprises a second semiconductor layerand a second gate electrode with a second gate insulating filminterposed therebetween, wherein a gradient of a side surface withrespect to a bottom surface of the first gate electrode is differentfrom that of a side surface with respect to a bottom surface of thesecond gate electrode, wherein the first n-channel thin film transistorcomprises a third semiconductor layer and a third gate electrode with athird gate insulating film interposed therebetween, and wherein agradient of a side surface with respect to a bottom surface of the thirdgate electrode is different from that of the side surface with respectto the bottom surface of the first gate electrode.
 2. The portableinformation terminal according to claim 1, wherein each of the firstgate electrode and the second gate electrode comprises a multiple-layerstructure.
 3. The portable information terminal according to claim 1,wherein each of the first gate electrode and the second gate electrodecomprises at least one selected from the group consisting of Ta, W, Ti,Mo, Al, Cu and Si.
 4. The portable information terminal according toclaim 1, wherein the first gate insulating film and the second gateinsulating film are not in contact with each other.
 5. The portableinformation terminal according to claim 1, wherein the portableinformation terminal is at least one selected from the group consistingof a mobile computer, a portable telephone and an electronic book.
 6. Aportable information terminal having a display device, the displaydevice comprising: a pixel portion formed over a substrate, the pixelportion comprising a first n-channel thin film transistor; and a drivercircuit portion formed over the substrate, the driver circuit portioncomprising a second n-channel thin film transistor and a p-channel thinfilm transistor, wherein the second n-channel thin film transistorcomprises a first semiconductor layer and a first gate electrode with afirst gate insulating film interposed therebetween, wherein thep-channel thin film transistor comprises a second semiconductor layerand a second gate electrode with a second gate insulating filminterposed therebetween, wherein a gradient of a side surface withrespect to a bottom surface of the second gate electrode is larger thanthat of a side surface with respect to a bottom surface of the firstgate electrode, wherein the first n-channel thin film transistorcomprises a third semiconductor layer and a third gate electrode with athird gate insulating film interposed therebetween, and wherein agradient of a side surface with respect to a bottom surface of the thirdgate electrode is larger than that of the side surface with respect tothe bottom surface of the first gate electrode.
 7. The portableinformation terminal according to claim 6, wherein each of the firstgate electrode and the second gate electrode comprises a multiple-layerstructure.
 8. The portable information terminal according to claim 6,wherein each of the first gate electrode and the second gate electrodecomprises at least one selected from the group consisting of Ta, W, Ti,Mo, Al, Cu and Si.
 9. The portable information terminal according toclaim 6, wherein the first gate insulating film and the second gateinsulating film are not in contact with each other.
 10. The portableinformation terminal according to claim 6, wherein the portableinformation terminal is at least one selected from the group consistingof a mobile computer, a portable telephone and an electronic book.
 11. Aportable information terminal having a display device, the displaydevice comprising: a pixel portion formed over a substrate, the pixelportion comprising a first n-channel thin film transistor; and a drivercircuit portion formed over the substrate, the driver circuit portioncomprising a second n-channel thin film transistor and a p-channel thinfilm transistor, wherein the second n-channel thin film transistorcomprises a first semiconductor layer and a first gate electrode with afirst gate insulating film interposed therebetween, wherein thep-channel thin film transistor comprises a second semiconductor layerand a second gate electrode with a second gate insulating filminterposed therebetween, wherein a bottom surface of the first gateelectrode is wider than that of the second gate electrode, wherein thefirst n-channel thin film transistor comprises a third semiconductorlayer and a third gate electrode with a third gate insulating filminterposed therebetween, and wherein the bottom surface of the firstgate electrode is wider than that of the third gate electrode.
 12. Theportable information terminal according to claim 11, wherein each of thefirst gate electrode and the second gate electrode comprises amultiple-layer structure.
 13. The portable information terminalaccording to claim 11, wherein each of the first gate electrode and thesecond gate electrode comprises at least one selected from the groupconsisting of Ta, W, Ti, Mo, Al, Cu and Si.
 14. The portable informationterminal according to claim 11, wherein the first gate insulating filmand the second gate insulating film are not in contact with each other.15. The portable information terminal according to claim 11, wherein theportable information terminal is at least one selected from the groupconsisting of a mobile computer, a portable telephone and an electronicbook.
 16. A portable information terminal having a display device, thedisplay device comprising: a pixel portion formed over a substrate, thepixel portion comprising a first n-channel thin film transistor; and adriver circuit portion formed over the substrate, the driver circuitportion comprising a second n-channel thin film transistor and ap-channel thin film transistor, wherein the second n-channel thin filmtransistor comprises a first semiconductor layer, a first gateinsulating film on the first semiconductor layer, and a first gateelectrode on the first gate insulating film, wherein the p-channel thinfilm transistor comprises a second semiconductor layer, a second gateinsulating film on the second semiconductor layer, and a second gateelectrode on the second gate insulating film, wherein a gradient of aside surface with respect to a bottom surface of the first gateelectrode is different from that of a side surface with respect to abottom surface of the second gate electrode, wherein the first n-channelthin film transistor comprises a third semiconductor layer, a third gateinsulating film on the third semiconductor layer, and a third gateelectrode on the third gate insulating film, and wherein a gradient of aside surface with respect to a bottom surface of the third gateelectrode is different from that of the side surface with respect to thebottom surface of the first gate electrode.
 17. The portable informationterminal according to claim 16, wherein each of the first gate electrodeand the second gate electrode comprises a multiple-layer structure. 18.The portable information terminal according to claim 16, wherein each ofthe first gate electrode and the second gate electrode comprises atleast one selected from the group consisting of Ta, W, Ti, Mo, Al, Cuand Si.
 19. The portable information terminal according to claim 16,wherein the first gate insulating film and the second gate insulatingfilm are not in contact with each other.
 20. The portable informationterminal according to claim 16, wherein the portable informationterminal is at least one selected from the group consisting of a mobilecomputer, a portable telephone and an electronic book.
 21. A portableinformation terminal having a display device, the display devicecomprising: a pixel portion formed over a substrate, the pixel portioncomprising a first n-channel thin film transistor; and a driver circuitportion formed over the substrate, the driver circuit portion comprisinga second n-channel thin film transistor and a p-channel thin filmtransistor, wherein the second n-channel thin film transistor comprisesa first semiconductor layer, a first gate insulating film on the firstsemiconductor layer, and a first gate electrode on the first gateinsulating film, wherein the p-channel thin film transistor comprises asecond semiconductor layer, a second gate insulating film on the secondsemiconductor layer, and a second gate electrode on the second gateinsulating film, wherein a gradient of a side surface with respect to abottom surface of the second gate electrode is larger than that of aside surface with respect to a bottom surface of the first gateelectrode, wherein the first n-channel thin film transistor comprises athird semiconductor layer, a third gate insulating film on the thirdsemiconductor layer, and a third gate electrode on the third gateinsulating film, and wherein a gradient of a side surface with respectto a bottom surface of the third gate electrode is larger than that ofthe side surface with respect to the bottom surface of the first gateelectrode.
 22. The portable information terminal according to claim 21,wherein each of the first gate electrode and the second gate electrodecomprises a multiple-layer structure.
 23. The portable informationterminal according to claim 21, wherein each of the first gate electrodeand the second gate electrode comprises at least one selected from thegroup consisting of Ta, W, Ti, Mo, Al, Cu and Si.
 24. The portableinformation terminal according to claim 21, wherein the first gateinsulating film and the second gate insulating film are not in contactwith each other.
 25. The portable information terminal according toclaim 21, wherein the portable information terminal is at least oneselected from the group consisting of a mobile computer, a portabletelephone and an electronic book.
 26. A portable information terminalhaving a display device, the display device comprising: a pixel portionformed over a substrate, the pixel portion comprising a first n-channelthin film transistor; and a driver circuit portion formed over thesubstrate, the driver circuit portion comprising a second n-channel thinfilm transistor and a p-channel thin film transistor, wherein the secondn-channel thin film transistor comprises a first semiconductor layer, afirst gate insulating film on the first semiconductor layer, and a firstgate electrode on the first gate insulating film, wherein the p-channelthin film transistor comprises a second semiconductor layer, a secondgate insulating film on the second semiconductor layer, and a secondgate electrode on the second gate insulating film, wherein a bottomsurface of the first gate electrode is wider than that of the secondgate electrode, wherein the first n-channel thin film transistorcomprises a third semiconductor layer, a third gate insulating film onthe third semiconductor layer, and a third gate electrode on the thirdgate insulating film, and wherein the bottom surface of the first gateelectrode is wider than that of the third gate electrode.
 27. Theportable information terminal according to claim 26, wherein each of thefirst gate electrode and the second gate electrode comprises amultiple-layer structure.
 28. The portable information terminalaccording to claim 26, wherein each of the first gate electrode and thesecond gate electrode comprises at least one selected from the groupconsisting of Ta, W, Ti, Mo, Al, Cu and Si.
 29. The portable informationterminal according to claim 26, wherein the first gate insulating filmand the second gate insulating film are not in contact with each other.30. The portable information terminal according to claim 26, wherein theportable information terminal is at least one selected from the groupconsisting of a mobile computer, a portable telephone and an electronicbook.